# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το django.db.models.fields

    # -*- coding: utf-8 -*-
    from __future__ import unicode_literals

    import collections
    import copy
    import datetime
    import decimal
    import itertools
    import uuid
    import warnings
    from base64 import b64decode, b64encode
    from functools import total_ordering

    from django import forms
    from django.apps import apps
    from django.conf import settings
    from django.core import checks, exceptions, validators
    # When the _meta object was formalized, this exception was moved to
    # django.core.exceptions. It is retained here for backwards compatibility
    # purposes.
    from django.core.exceptions import FieldDoesNotExist  # NOQA
    from django.db import connection, connections, router
    from django.db.models.query_utils import DeferredAttribute, RegisterLookupMixin
    from django.utils import six, timezone
    from django.utils.datastructures import DictWrapper
    from django.utils.dateparse import (
        parse_date, parse_datetime, parse_duration, parse_time,
    )
    from django.utils.deprecation import (
        RemovedInDjango20Warning, warn_about_renamed_method,
    )
    from django.utils.duration import duration_string
    from django.utils.encoding import (
        force_bytes, force_text, python_2_unicode_compatible, smart_text,
    )
    from django.utils.functional import Promise, cached_property, curry
    from django.utils.ipv6 import clean_ipv6_address
    from django.utils.itercompat import is_iterable
    from django.utils.text import capfirst
    from django.utils.translation import ugettext_lazy as _

    # Avoid "TypeError: Item in ``from list'' not a string" -- unicode_literals
    # makes these strings unicode
    __all__ = [str(x) for x in (
        'AutoField', 'BLANK_CHOICE_DASH', 'BigAutoField', 'BigIntegerField',
        'BinaryField', 'BooleanField', 'CharField', 'CommaSeparatedIntegerField',
        'DateField', 'DateTimeField', 'DecimalField', 'DurationField',
        'EmailField', 'Empty', 'Field', 'FieldDoesNotExist', 'FilePathField',
        'FloatField', 'GenericIPAddressField', 'IPAddressField', 'IntegerField',
        'NOT_PROVIDED', 'NullBooleanField', 'PositiveIntegerField',
        'PositiveSmallIntegerField', 'SlugField', 'SmallIntegerField', 'TextField',
        'TimeField', 'URLField', 'UUIDField',
    )]

    class Empty(object):
        pass

    class NOT_PROVIDED:
        pass

    # The values to use for "blank" in SelectFields. Will be appended to the start
    # of most "choices" lists.
    BLANK_CHOICE_DASH = [("", "---------")]

    def _load_field(app_label, model_name, field_name):
        return apps.get_model(app_label, model_name)._meta.get_field(field_name)

    # A guide to Field parameters:
    #
    #   * name:      The name of the field specified in the model.
    #   * attname:   The attribute to use on the model object. This is the same as
    #                "name", except in the case of ForeignKeys, where "_id" is
    #                appended.
    #   * db_column: The db_column specified in the model (or None).
    #   * column:    The database column for this field. This is the same as
    #                "attname", except if db_column is specified.
    #
    # Code that introspects values, or does other dynamic things, should use
    # attname. For example, this gets the primary key value of object "obj":
    #
    #     getattr(obj, opts.pk.attname)

    def _empty(of_cls):
        new = Empty()
        new.__class__ = of_cls
        return new

    @total_ordering
    @python_2_unicode_compatible
    class Field(RegisterLookupMixin):
        """Base class for all field types"""

        # Designates whether empty strings fundamentally are allowed at the
        # database level.
        empty_strings_allowed = True
        empty_values = list(validators.EMPTY_VALUES)

        # These track each time a Field instance is created. Used to retain order.
        # The auto_creation_counter is used for fields that Django implicitly
        # creates, creation_counter is used for all user-specified fields.
        creation_counter = 0
        auto_creation_counter = -1
        default_validators = []  # Default set of validators
        default_error_messages = {
            'invalid_choice': _('Value %(value)r is not a valid choice.'),
            'null': _('This field cannot be null.'),
            'blank': _('This field cannot be blank.'),
            'unique': _('%(model_name)s with this %(field_label)s '
                        'already exists.'),
            # Translators: The 'lookup_type' is one of 'date', 'year' or 'month'.
            # Eg: "Title must be unique for pub_date year"
            'unique_for_date': _("%(field_label)s must be unique for "
                                 "%(date_field_label)s %(lookup_type)s."),
        }
        system_check_deprecated_details = None
        system_check_removed_details = None

        # Field flags
        hidden = False

        many_to_many = None
        many_to_one = None
        one_to_many = None
        one_to_one = None
        related_model = None

        # Generic field type description, usually overridden by subclasses
        def _description(self):
            return _('Field of type: %(field_type)s') % {
                'field_type': self.__class__.__name__
            }
        description = property(_description)

        def __init__(self, verbose_name=None, name=None, primary_key=False,
                     max_length=None, unique=False, blank=False, null=False,
                     db_index=False, rel=None, default=NOT_PROVIDED, editable=True,
                     serialize=True, unique_for_date=None, unique_for_month=None,
                     unique_for_year=None, choices=None, help_text='', db_column=None,
                     db_tablespace=None, auto_created=False, validators=[],
                     error_messages=None):
            self.name = name
            self.verbose_name = verbose_name  # May be set by set_attributes_from_name
            self._verbose_name = verbose_name  # Store original for deconstruction
            self.primary_key = primary_key
            self.max_length, self._unique = max_length, unique
            self.blank, self.null = blank, null
            self.remote_field = rel
            self.is_relation = self.remote_field is not None
            self.default = default
            self.editable = editable
            self.serialize = serialize
            self.unique_for_date = unique_for_date
            self.unique_for_month = unique_for_month
            self.unique_for_year = unique_for_year
            if isinstance(choices, collections.Iterator):
                choices = list(choices)
            self.choices = choices or []
            self.help_text = help_text
            self.db_index = db_index
            self.db_column = db_column
            self.db_tablespace = db_tablespace or settings.DEFAULT_INDEX_TABLESPACE
            self.auto_created = auto_created

            # Adjust the appropriate creation counter, and save our local copy.
            if auto_created:
                self.creation_counter = Field.auto_creation_counter
                Field.auto_creation_counter -= 1
            else:
                self.creation_counter = Field.creation_counter
                Field.creation_counter += 1

            self._validators = validators  # Store for deconstruction later

            messages = {}
            for c in reversed(self.__class__.__mro__):
                messages.update(getattr(c, 'default_error_messages', {}))
            messages.update(error_messages or {})
            self._error_messages = error_messages  # Store for deconstruction later
            self.error_messages = messages

        def __str__(self):
            """ Return "app_label.model_label.field_name". """
            model = self.model
            app = model._meta.app_label
            return '%s.%s.%s' % (app, model._meta.object_name, self.name)

        def __repr__(self):
            """
            Displays the module, class and name of the field.
            """
            path = '%s.%s' % (self.__class__.__module__, self.__class__.__name__)
            name = getattr(self, 'name', None)
            if name is not None:
                return '<%s: %s>' % (path, name)
            return '<%s>' % path

        def check(self, **kwargs):
            errors = []
            errors.extend(self._check_field_name())
            errors.extend(self._check_choices())
            errors.extend(self._check_db_index())
            errors.extend(self._check_null_allowed_for_primary_keys())
            errors.extend(self._check_backend_specific_checks(**kwargs))
            errors.extend(self._check_deprecation_details())
            return errors

        def _check_field_name(self):
            """ Check if field name is valid, i.e. 1) does not end with an
            underscore, 2) does not contain "__" and 3) is not "pk". """

            if self.name.endswith('_'):
                return [
                    checks.Error(
                        'Field names must not end with an underscore.',
                        obj=self,
                        id='fields.E001',
                    )
                ]
            elif '__' in self.name:
                return [
                    checks.Error(
                        'Field names must not contain "__".',
                        obj=self,
                        id='fields.E002',
                    )
                ]
            elif self.name == 'pk':
                return [
                    checks.Error(
                        "'pk' is a reserved word that cannot be used as a field name.",
                        obj=self,
                        id='fields.E003',
                    )
                ]
            else:
                return []

        @property
        def rel(self):
            warnings.warn(
                "Usage of field.rel has been deprecated. Use field.remote_field instead.",
                RemovedInDjango20Warning, 2)
            return self.remote_field

        def _check_choices(self):
            if self.choices:
                if (isinstance(self.choices, six.string_types) or
                        not is_iterable(self.choices)):
                    return [
                        checks.Error(
                            "'choices' must be an iterable (e.g., a list or tuple).",
                            obj=self,
                            id='fields.E004',
                        )
                    ]
                elif any(isinstance(choice, six.string_types) or
                         not is_iterable(choice) or len(choice) != 2
                         for choice in self.choices):
                    return [
                        checks.Error(
                            "'choices' must be an iterable containing "
                            "(actual value, human readable name) tuples.",
                            obj=self,
                            id='fields.E005',
                        )
                    ]
                else:
                    return []
            else:
                return []

        def _check_db_index(self):
            if self.db_index not in (None, True, False):
                return [
                    checks.Error(
                        "'db_index' must be None, True or False.",
                        obj=self,
                        id='fields.E006',
                    )
                ]
            else:
                return []

        def _check_null_allowed_for_primary_keys(self):
            if (self.primary_key and self.null and
                    not connection.features.interprets_empty_strings_as_nulls):
                # We cannot reliably check this for backends like Oracle which
                # consider NULL and '' to be equal (and thus set up
                # character-based fields a little differently).
                return [
                    checks.Error(
                        'Primary keys must not have null=True.',
                        hint=('Set null=False on the field, or '
                              'remove primary_key=True argument.'),
                        obj=self,
                        id='fields.E007',
                    )
                ]
            else:
                return []

        def _check_backend_specific_checks(self, **kwargs):
            app_label = self.model._meta.app_label
            for db in connections:
                if router.allow_migrate(db, app_label, model_name=self.model._meta.model_name):
                    return connections[db].validation.check_field(self, **kwargs)
            return []

        def _check_deprecation_details(self):
            if self.system_check_removed_details is not None:
                return [
                    checks.Error(
                        self.system_check_removed_details.get(
                            'msg',
                            '%s has been removed except for support in historical '
                            'migrations.' % self.__class__.__name__
                        ),
                        hint=self.system_check_removed_details.get('hint'),
                        obj=self,
                        id=self.system_check_removed_details.get('id', 'fields.EXXX'),
                    )
                ]
            elif self.system_check_deprecated_details is not None:
                return [
                    checks.Warning(
                        self.system_check_deprecated_details.get(
                            'msg',
                            '%s has been deprecated.' % self.__class__.__name__
                        ),
                        hint=self.system_check_deprecated_details.get('hint'),
                        obj=self,
                        id=self.system_check_deprecated_details.get('id', 'fields.WXXX'),
                    )
                ]
            return []

        def get_col(self, alias, output_field=None):
            if output_field is None:
                output_field = self
            if alias != self.model._meta.db_table or output_field != self:
                from django.db.models.expressions import Col
                return Col(alias, self, output_field)
            else:
                return self.cached_col

        @cached_property
        def cached_col(self):
            from django.db.models.expressions import Col
            return Col(self.model._meta.db_table, self)

        def select_format(self, compiler, sql, params):
            """
            Custom format for select clauses. For example, GIS columns need to be
            selected as AsText(table.col) on MySQL as the table.col data can't be used
            by Django.
            """
            return sql, params

        def deconstruct(self):
            """
            Returns enough information to recreate the field as a 4-tuple:

             * The name of the field on the model, if contribute_to_class has been run
             * The import path of the field, including the class: django.db.models.IntegerField
               This should be the most portable version, so less specific may be better.
             * A list of positional arguments
             * A dict of keyword arguments

            Note that the positional or keyword arguments must contain values of the
            following types (including inner values of collection types):

             * None, bool, str, unicode, int, long, float, complex, set, frozenset, list, tuple, dict
             * UUID
             * datetime.datetime (naive), datetime.date
             * top-level classes, top-level functions - will be referenced by their full import path
             * Storage instances - these have their own deconstruct() method

            This is because the values here must be serialized into a text format
            (possibly new Python code, possibly JSON) and these are the only types
            with encoding handlers defined.

            There's no need to return the exact way the field was instantiated this time,
            just ensure that the resulting field is the same - prefer keyword arguments
            over positional ones, and omit parameters with their default values.
            """
            # Short-form way of fetching all the default parameters
            keywords = {}
            possibles = {
                "verbose_name": None,
                "primary_key": False,
                "max_length": None,
                "unique": False,
                "blank": False,
                "null": False,
                "db_index": False,
                "default": NOT_PROVIDED,
                "editable": True,
                "serialize": True,
                "unique_for_date": None,
                "unique_for_month": None,
                "unique_for_year": None,
                "choices": [],
                "help_text": '',
                "db_column": None,
                "db_tablespace": settings.DEFAULT_INDEX_TABLESPACE,
                "auto_created": False,
                "validators": [],
                "error_messages": None,
            }
            attr_overrides = {
                "unique": "_unique",
                "error_messages": "_error_messages",
                "validators": "_validators",
                "verbose_name": "_verbose_name",
            }
            equals_comparison = {"choices", "validators", "db_tablespace"}
            for name, default in possibles.items():
                value = getattr(self, attr_overrides.get(name, name))
                # Unroll anything iterable for choices into a concrete list
                if name == "choices" and isinstance(value, collections.Iterable):
                    value = list(value)
                # Do correct kind of comparison
                if name in equals_comparison:
                    if value != default:
                        keywords[name] = value
                else:
                    if value is not default:
                        keywords[name] = value
            # Work out path - we shorten it for known Django core fields
            path = "%s.%s" % (self.__class__.__module__, self.__class__.__name__)
            if path.startswith("django.db.models.fields.related"):
                path = path.replace("django.db.models.fields.related", "django.db.models")
            if path.startswith("django.db.models.fields.files"):
                path = path.replace("django.db.models.fields.files", "django.db.models")
            if path.startswith("django.db.models.fields.proxy"):
                path = path.replace("django.db.models.fields.proxy", "django.db.models")
            if path.startswith("django.db.models.fields"):
                path = path.replace("django.db.models.fields", "django.db.models")
            # Return basic info - other fields should override this.
            return (
                force_text(self.name, strings_only=True),
                path,
                [],
                keywords,
            )

        def clone(self):
            """
            Uses deconstruct() to clone a new copy of this Field.
            Will not preserve any class attachments/attribute names.
            """
            name, path, args, kwargs = self.deconstruct()
            return self.__class__(*args, **kwargs)

        def __eq__(self, other):
            # Needed for @total_ordering
            if isinstance(other, Field):
                return self.creation_counter == other.creation_counter
            return NotImplemented

        def __lt__(self, other):
            # This is needed because bisect does not take a comparison function.
            if isinstance(other, Field):
                return self.creation_counter < other.creation_counter
            return NotImplemented

        def __hash__(self):
            return hash(self.creation_counter)

        def __deepcopy__(self, memodict):
            # We don't have to deepcopy very much here, since most things are not
            # intended to be altered after initial creation.
            obj = copy.copy(self)
            if self.remote_field:
                obj.remote_field = copy.copy(self.remote_field)
                if hasattr(self.remote_field, 'field') and self.remote_field.field is self:
                    obj.remote_field.field = obj
            memodict[id(self)] = obj
            return obj

        def __copy__(self):
            # We need to avoid hitting __reduce__, so define this
            # slightly weird copy construct.
            obj = Empty()
            obj.__class__ = self.__class__
            obj.__dict__ = self.__dict__.copy()
            return obj

        def __reduce__(self):
            """
            Pickling should return the model._meta.fields instance of the field,
            not a new copy of that field. So, we use the app registry to load the
            model and then the field back.
            """
            if not hasattr(self, 'model'):
                # Fields are sometimes used without attaching them to models (for
                # example in aggregation). In this case give back a plain field
                # instance. The code below will create a new empty instance of
                # class self.__class__, then update its dict with self.__dict__
                # values - so, this is very close to normal pickle.
                return _empty, (self.__class__,), self.__dict__
            return _load_field, (self.model._meta.app_label, self.model._meta.object_name,
                                 self.name)

        def get_pk_value_on_save(self, instance):
            """
            Hook to generate new PK values on save. This method is called when
            saving instances with no primary key value set. If this method returns
            something else than None, then the returned value is used when saving
            the new instance.
            """
            if self.default:
                return self.get_default()
            return None

        def to_python(self, value):
            """
            Converts the input value into the expected Python data type, raising
            django.core.exceptions.ValidationError if the data can't be converted.
            Returns the converted value. Subclasses should override this.
            """
            return value

        @cached_property
        def validators(self):
            """
            Some validators can't be created at field initialization time.
            This method provides a way to delay their creation until required.
            """
            return list(itertools.chain(self.default_validators, self._validators))

        def run_validators(self, value):
            if value in self.empty_values:
                return

            errors = []
            for v in self.validators:
                try:
                    v(value)
                except exceptions.ValidationError as e:
                    if hasattr(e, 'code') and e.code in self.error_messages:
                        e.message = self.error_messages[e.code]
                    errors.extend(e.error_list)

            if errors:
                raise exceptions.ValidationError(errors)

        def validate(self, value, model_instance):
            """
            Validates value and throws ValidationError. Subclasses should override
            this to provide validation logic.
            """
            if not self.editable:
                # Skip validation for non-editable fields.
                return

            if self.choices and value not in self.empty_values:
                for option_key, option_value in self.choices:
                    if isinstance(option_value, (list, tuple)):
                        # This is an optgroup, so look inside the group for
                        # options.
                        for optgroup_key, optgroup_value in option_value:
                            if value == optgroup_key:
                                return
                    elif value == option_key:
                        return
                raise exceptions.ValidationError(
                    self.error_messages['invalid_choice'],
                    code='invalid_choice',
                    params={'value': value},
                )

            if value is None and not self.null:
                raise exceptions.ValidationError(self.error_messages['null'], code='null')

            if not self.blank and value in self.empty_values:
                raise exceptions.ValidationError(self.error_messages['blank'], code='blank')

        def clean(self, value, model_instance):
            """
            Convert the value's type and run validation. Validation errors
            from to_python and validate are propagated. The correct value is
            returned if no error is raised.
            """
            value = self.to_python(value)
            self.validate(value, model_instance)
            self.run_validators(value)
            return value

        def db_check(self, connection):
            """
            Return the database column check constraint for this field, for the
            provided connection. Works the same way as db_type() for the case that
            get_internal_type() does not map to a preexisting model field.
            """
            data = DictWrapper(self.__dict__, connection.ops.quote_name, "qn_")
            try:
                return connection.data_type_check_constraints[self.get_internal_type()] % data
            except KeyError:
                return None

        def db_type(self, connection):
            """
            Return the database column data type for this field, for the provided
            connection.
            """
            # The default implementation of this method looks at the
            # backend-specific data_types dictionary, looking up the field by its
            # "internal type".
            #
            # A Field class can implement the get_internal_type() method to specify
            # which *preexisting* Django Field class it's most similar to -- i.e.,
            # a custom field might be represented by a TEXT column type, which is
            # the same as the TextField Django field type, which means the custom
            # field's get_internal_type() returns 'TextField'.
            #
            # But the limitation of the get_internal_type() / data_types approach
            # is that it cannot handle database column types that aren't already
            # mapped to one of the built-in Django field types. In this case, you
            # can implement db_type() instead of get_internal_type() to specify
            # exactly which wacky database column type you want to use.
            data = DictWrapper(self.__dict__, connection.ops.quote_name, "qn_")
            try:
                return connection.data_types[self.get_internal_type()] % data
            except KeyError:
                return None

        def rel_db_type(self, connection):
            """
            Return the data type that a related field pointing to this field should
            use. For example, this method is called by ForeignKey and OneToOneField
            to determine its data type.
            """
            return self.db_type(connection)

        def db_parameters(self, connection):
            """
            Extension of db_type(), providing a range of different return
            values (type, checks).
            This will look at db_type(), allowing custom model fields to override it.
            """
            type_string = self.db_type(connection)
            check_string = self.db_check(connection)
            return {
                "type": type_string,
                "check": check_string,
            }

        def db_type_suffix(self, connection):
            return connection.data_types_suffix.get(self.get_internal_type())

        def get_db_converters(self, connection):
            if hasattr(self, 'from_db_value'):
                return [self.from_db_value]
            return []

        @property
        def unique(self):
            return self._unique or self.primary_key

        def set_attributes_from_name(self, name):
            if not self.name:
                self.name = name
            self.attname, self.column = self.get_attname_column()
            self.concrete = self.column is not None
            if self.verbose_name is None and self.name:
                self.verbose_name = self.name.replace('_', ' ')

        def contribute_to_class(self, cls, name, private_only=False, virtual_only=NOT_PROVIDED):
            """
            Register the field with the model class it belongs to.

            If private_only is True, a separate instance of this field will be
            created for every subclass of cls, even if cls is not an abstract
            model.
            """
            if virtual_only is not NOT_PROVIDED:
                warnings.warn(
                    "The `virtual_only` argument of Field.contribute_to_class() "
                    "has been renamed to `private_only`.",
                    RemovedInDjango20Warning, stacklevel=2
                )
                private_only = virtual_only
            self.set_attributes_from_name(name)
            self.model = cls
            if private_only:
                cls._meta.add_field(self, private=True)
            else:
                cls._meta.add_field(self)
            if self.column:
                # Don't override classmethods with the descriptor. This means that
                # if you have a classmethod and a field with the same name, then
                # such fields can't be deferred (we don't have a check for this).
                if not getattr(cls, self.attname, None):
                    setattr(cls, self.attname, DeferredAttribute(self.attname, cls))
            if self.choices:
                setattr(cls, 'get_%s_display' % self.name,
                        curry(cls._get_FIELD_display, field=self))

        def get_filter_kwargs_for_object(self, obj):
            """
            Return a dict that when passed as kwargs to self.model.filter(), would
            yield all instances having the same value for this field as obj has.
            """
            return {self.name: getattr(obj, self.attname)}

        def get_attname(self):
            return self.name

        def get_attname_column(self):
            attname = self.get_attname()
            column = self.db_column or attname
            return attname, column

        def get_cache_name(self):
            return '_%s_cache' % self.name

        def get_internal_type(self):
            return self.__class__.__name__

        def pre_save(self, model_instance, add):
            """
            Returns field's value just before saving.
            """
            return getattr(model_instance, self.attname)

        def get_prep_value(self, value):
            """
            Perform preliminary non-db specific value checks and conversions.
            """
            if isinstance(value, Promise):
                value = value._proxy____cast()
            return value

        def get_db_prep_value(self, value, connection, prepared=False):
            """Returns field's value prepared for interacting with the database
            backend.

            Used by the default implementations of get_db_prep_save().
            """
            if not prepared:
                value = self.get_prep_value(value)
            return value

        def get_db_prep_save(self, value, connection):
            """
            Returns field's value prepared for saving into a database.
            """
            return self.get_db_prep_value(value, connection=connection,
                                          prepared=False)

        def has_default(self):
            """
            Returns a boolean of whether this field has a default value.
            """
            return self.default is not NOT_PROVIDED

        def get_default(self):
            """
            Returns the default value for this field.
            """
            if self.has_default():
                if callable(self.default):
                    return self.default()
                return self.default
            if not self.empty_strings_allowed or self.null and not connection.features.interprets_empty_strings_as_nulls:
                return None
            return ""

        def get_choices(self, include_blank=True, blank_choice=BLANK_CHOICE_DASH, limit_choices_to=None):
            """Returns choices with a default blank choices included, for use
            as SelectField choices for this field."""
            blank_defined = False
            choices = list(self.choices) if self.choices else []
            named_groups = choices and isinstance(choices[0][1], (list, tuple))
            if not named_groups:
                for choice, __ in choices:
                    if choice in ('', None):
                        blank_defined = True
                        break

            first_choice = (blank_choice if include_blank and
                            not blank_defined else [])
            if self.choices:
                return first_choice + choices
            rel_model = self.remote_field.model
            limit_choices_to = limit_choices_to or self.get_limit_choices_to()
            if hasattr(self.remote_field, 'get_related_field'):
                lst = [(getattr(x, self.remote_field.get_related_field().attname),
                       smart_text(x))
                       for x in rel_model._default_manager.complex_filter(
                           limit_choices_to)]
            else:
                lst = [(x._get_pk_val(), smart_text(x))
                       for x in rel_model._default_manager.complex_filter(
                           limit_choices_to)]
            return first_choice + lst

        @warn_about_renamed_method(
            'Field', '_get_val_from_obj', 'value_from_object',
            RemovedInDjango20Warning
        )
        def _get_val_from_obj(self, obj):
            if obj is not None:
                return getattr(obj, self.attname)
            else:
                return self.get_default()

        def value_to_string(self, obj):
            """
            Returns a string value of this field from the passed obj.
            This is used by the serialization framework.
            """
            return smart_text(self.value_from_object(obj))

        def _get_flatchoices(self):
            """Flattened version of choices tuple."""
            flat = []
            for choice, value in self.choices:
                if isinstance(value, (list, tuple)):
                    flat.extend(value)
                else:
                    flat.append((choice, value))
            return flat
        flatchoices = property(_get_flatchoices)

        def save_form_data(self, instance, data):
            setattr(instance, self.name, data)

        def formfield(self, form_class=None, choices_form_class=None, **kwargs):
            """
            Returns a django.forms.Field instance for this database Field.
            """
            defaults = {'required': not self.blank,
                        'label': capfirst(self.verbose_name),
                        'help_text': self.help_text}
            if self.has_default():
                if callable(self.default):
                    defaults['initial'] = self.default
                    defaults['show_hidden_initial'] = True
                else:
                    defaults['initial'] = self.get_default()
            if self.choices:
                # Fields with choices get special treatment.
                include_blank = (self.blank or
                                 not (self.has_default() or 'initial' in kwargs))
                defaults['choices'] = self.get_choices(include_blank=include_blank)
                defaults['coerce'] = self.to_python
                if self.null:
                    defaults['empty_value'] = None
                if choices_form_class is not None:
                    form_class = choices_form_class
                else:
                    form_class = forms.TypedChoiceField
                # Many of the subclass-specific formfield arguments (min_value,
                # max_value) don't apply for choice fields, so be sure to only pass
                # the values that TypedChoiceField will understand.
                for k in list(kwargs):
                    if k not in ('coerce', 'empty_value', 'choices', 'required',
                                 'widget', 'label', 'initial', 'help_text',
                                 'error_messages', 'show_hidden_initial'):
                        del kwargs[k]
            defaults.update(kwargs)
            if form_class is None:
                form_class = forms.CharField
            return form_class(**defaults)

        def value_from_object(self, obj):
            """
            Returns the value of this field in the given model instance.
            """
            return getattr(obj, self.attname)

    class AutoField(Field):
        description = _("Integer")

        empty_strings_allowed = False
        default_error_messages = {
            'invalid': _("'%(value)s' value must be an integer."),
        }

        def __init__(self, *args, **kwargs):
            kwargs['blank'] = True
            super(AutoField, self).__init__(*args, **kwargs)

        def check(self, **kwargs):
            errors = super(AutoField, self).check(**kwargs)
            errors.extend(self._check_primary_key())
            return errors

        def _check_primary_key(self):
            if not self.primary_key:
                return [
                    checks.Error(
                        'AutoFields must set primary_key=True.',
                        obj=self,
                        id='fields.E100',
                    ),
                ]
            else:
                return []

        def deconstruct(self):
            name, path, args, kwargs = super(AutoField, self).deconstruct()
            del kwargs['blank']
            kwargs['primary_key'] = True
            return name, path, args, kwargs

        def get_internal_type(self):
            return "AutoField"

        def to_python(self, value):
            if value is None:
                return value
            try:
                return int(value)
            except (TypeError, ValueError):
                raise exceptions.ValidationError(
                    self.error_messages['invalid'],
                    code='invalid',
                    params={'value': value},
                )

        def rel_db_type(self, connection):
            return IntegerField().db_type(connection=connection)

        def validate(self, value, model_instance):
            pass

        def get_db_prep_value(self, value, connection, prepared=False):
            if not prepared:
                value = self.get_prep_value(value)
                value = connection.ops.validate_autopk_value(value)
            return value

        def get_prep_value(self, value):
            value = super(AutoField, self).get_prep_value(value)
            if value is None:
                return None
            return int(value)

        def contribute_to_class(self, cls, name, **kwargs):
            assert not cls._meta.has_auto_field, \
                "A model can't have more than one AutoField."
            super(AutoField, self).contribute_to_class(cls, name, **kwargs)
            cls._meta.has_auto_field = True
            cls._meta.auto_field = self

        def formfield(self, **kwargs):
            return None

    class BigAutoField(AutoField):
        description = _("Big (8 byte) integer")

        def get_internal_type(self):
            return "BigAutoField"

        def rel_db_type(self, connection):
            return BigIntegerField().db_type(connection=connection)

    class BooleanField(Field):
        empty_strings_allowed = False
        default_error_messages = {
            'invalid': _("'%(value)s' value must be either True or False."),
        }
        description = _("Boolean (Either True or False)")

        def __init__(self, *args, **kwargs):
            kwargs['blank'] = True
            super(BooleanField, self).__init__(*args, **kwargs)

        def check(self, **kwargs):
            errors = super(BooleanField, self).check(**kwargs)
            errors.extend(self._check_null(**kwargs))
            return errors

        def _check_null(self, **kwargs):
            if getattr(self, 'null', False):
                return [
                    checks.Error(
                        'BooleanFields do not accept null values.',
                        hint='Use a NullBooleanField instead.',
                        obj=self,
                        id='fields.E110',
                    )
                ]
            else:
                return []

        def deconstruct(self):
            name, path, args, kwargs = super(BooleanField, self).deconstruct()
            del kwargs['blank']
            return name, path, args, kwargs

        def get_internal_type(self):
            return "BooleanField"

        def to_python(self, value):
            if value in (True, False):
                # if value is 1 or 0 than it's equal to True or False, but we want
                # to return a true bool for semantic reasons.
                return bool(value)
            if value in ('t', 'True', '1'):
                return True
            if value in ('f', 'False', '0'):
                return False
            raise exceptions.ValidationError(
                self.error_messages['invalid'],
                code='invalid',
                params={'value': value},
            )

        def get_prep_value(self, value):
            value = super(BooleanField, self).get_prep_value(value)
            if value is None:
                return None
            return self.to_python(value)

        def formfield(self, **kwargs):
            # Unlike most fields, BooleanField figures out include_blank from
            # self.null instead of self.blank.
            if self.choices:
                include_blank = not (self.has_default() or 'initial' in kwargs)
                defaults = {'choices': self.get_choices(include_blank=include_blank)}
            else:
                defaults = {'form_class': forms.BooleanField}
            defaults.update(kwargs)
            return super(BooleanField, self).formfield(**defaults)

    class CharField(Field):
        description = _("String (up to %(max_length)s)")

        def __init__(self, *args, **kwargs):
            super(CharField, self).__init__(*args, **kwargs)
            self.validators.append(validators.MaxLengthValidator(self.max_length))

        def check(self, **kwargs):
            errors = super(CharField, self).check(**kwargs)
            errors.extend(self._check_max_length_attribute(**kwargs))
            return errors

        def _check_max_length_attribute(self, **kwargs):
            if self.max_length is None:
                return [
                    checks.Error(
                        "CharFields must define a 'max_length' attribute.",
                        obj=self,
                        id='fields.E120',
                    )
                ]
            elif not isinstance(self.max_length, six.integer_types) or self.max_length <= 0:
                return [
                    checks.Error(
                        "'max_length' must be a positive integer.",
                        obj=self,
                        id='fields.E121',
                    )
                ]
            else:
                return []

        def get_internal_type(self):
            return "CharField"

        def to_python(self, value):
            if isinstance(value, six.string_types) or value is None:
                return value
            return smart_text(value)

        def get_prep_value(self, value):
            value = super(CharField, self).get_prep_value(value)
            return self.to_python(value)

        def formfield(self, **kwargs):
            # Passing max_length to forms.CharField means that the value's length
            # will be validated twice. This is considered acceptable since we want
            # the value in the form field (to pass into widget for example).
            defaults = {'max_length': self.max_length}
            defaults.update(kwargs)
            return super(CharField, self).formfield(**defaults)

    class CommaSeparatedIntegerField(CharField):
        default_validators = [validators.validate_comma_separated_integer_list]
        description = _("Comma-separated integers")
        system_check_deprecated_details = {
            'msg': (
                'CommaSeparatedIntegerField has been deprecated. Support '
                'for it (except in historical migrations) will be removed '
                'in Django 2.0.'
            ),
            'hint': (
                'Use CharField(validators=[validate_comma_separated_integer_list]) instead.'
            ),
            'id': 'fields.W901',
        }

        def formfield(self, **kwargs):
            defaults = {
                'error_messages': {
                    'invalid': _('Enter only digits separated by commas.'),
                }
            }
            defaults.update(kwargs)
            return super(CommaSeparatedIntegerField, self).formfield(**defaults)

    class DateTimeCheckMixin(object):

        def check(self, **kwargs):
            errors = super(DateTimeCheckMixin, self).check(**kwargs)
            errors.extend(self._check_mutually_exclusive_options())
            errors.extend(self._check_fix_default_value())
            return errors

        def _check_mutually_exclusive_options(self):
            # auto_now, auto_now_add, and default are mutually exclusive
            # options. The use of more than one of these options together
            # will trigger an Error
            mutually_exclusive_options = [self.auto_now_add, self.auto_now, self.has_default()]
            enabled_options = [option not in (None, False) for option in mutually_exclusive_options].count(True)
            if enabled_options > 1:
                return [
                    checks.Error(
                        "The options auto_now, auto_now_add, and default "
                        "are mutually exclusive. Only one of these options "
                        "may be present.",
                        obj=self,
                        id='fields.E160',
                    )
                ]
            else:
                return []

        def _check_fix_default_value(self):
            return []

    class DateField(DateTimeCheckMixin, Field):
        empty_strings_allowed = False
        default_error_messages = {
            'invalid': _("'%(value)s' value has an invalid date format. It must be "
                         "in YYYY-MM-DD format."),
            'invalid_date': _("'%(value)s' value has the correct format (YYYY-MM-DD) "
                              "but it is an invalid date."),
        }
        description = _("Date (without time)")

        def __init__(self, verbose_name=None, name=None, auto_now=False,
                     auto_now_add=False, **kwargs):
            self.auto_now, self.auto_now_add = auto_now, auto_now_add
            if auto_now or auto_now_add:
                kwargs['editable'] = False
                kwargs['blank'] = True
            super(DateField, self).__init__(verbose_name, name, **kwargs)

        def _check_fix_default_value(self):
            """
            Adds a warning to the checks framework stating, that using an actual
            date or datetime value is probably wrong; it's only being evaluated on
            server start-up.

            For details see ticket #21905
            """
            if not self.has_default():
                return []

            now = timezone.now()
            if not timezone.is_naive(now):
                now = timezone.make_naive(now, timezone.utc)
            value = self.default
            if isinstance(value, datetime.datetime):
                if not timezone.is_naive(value):
                    value = timezone.make_naive(value, timezone.utc)
                value = value.date()
            elif isinstance(value, datetime.date):
                # Nothing to do, as dates don't have tz information
                pass
            else:
                # No explicit date / datetime value -- no checks necessary
                return []
            offset = datetime.timedelta(days=1)
            lower = (now - offset).date()
            upper = (now + offset).date()
            if lower <= value <= upper:
                return [
                    checks.Warning(
                        'Fixed default value provided.',
                        hint='It seems you set a fixed date / time / datetime '
                             'value as default for this field. This may not be '
                             'what you want. If you want to have the current date '
                             'as default, use `django.utils.timezone.now`',
                        obj=self,
                        id='fields.W161',
                    )
                ]

            return []

        def deconstruct(self):
            name, path, args, kwargs = super(DateField, self).deconstruct()
            if self.auto_now:
                kwargs['auto_now'] = True
            if self.auto_now_add:
                kwargs['auto_now_add'] = True
            if self.auto_now or self.auto_now_add:
                del kwargs['editable']
                del kwargs['blank']
            return name, path, args, kwargs

        def get_internal_type(self):
            return "DateField"

        def to_python(self, value):
            if value is None:
                return value
            if isinstance(value, datetime.datetime):
                if settings.USE_TZ and timezone.is_aware(value):
                    # Convert aware datetimes to the default time zone
                    # before casting them to dates (#17742).
                    default_timezone = timezone.get_default_timezone()
                    value = timezone.make_naive(value, default_timezone)
                return value.date()
            if isinstance(value, datetime.date):
                return value

            try:
                parsed = parse_date(value)
                if parsed is not None:
                    return parsed
            except ValueError:
                raise exceptions.ValidationError(
                    self.error_messages['invalid_date'],
                    code='invalid_date',
                    params={'value': value},
                )

            raise exceptions.ValidationError(
                self.error_messages['invalid'],
                code='invalid',
                params={'value': value},
            )

        def pre_save(self, model_instance, add):
            if self.auto_now or (self.auto_now_add and add):
                value = datetime.date.today()
                setattr(model_instance, self.attname, value)
                return value
            else:
                return super(DateField, self).pre_save(model_instance, add)

        def contribute_to_class(self, cls, name, **kwargs):
            super(DateField, self).contribute_to_class(cls, name, **kwargs)
            if not self.null:
                setattr(
                    cls, 'get_next_by_%s' % self.name,
                    curry(cls._get_next_or_previous_by_FIELD, field=self, is_next=True)
                )
                setattr(
                    cls, 'get_previous_by_%s' % self.name,
                    curry(cls._get_next_or_previous_by_FIELD, field=self, is_next=False)
                )

        def get_prep_value(self, value):
            value = super(DateField, self).get_prep_value(value)
            return self.to_python(value)

        def get_db_prep_value(self, value, connection, prepared=False):
            # Casts dates into the format expected by the backend
            if not prepared:
                value = self.get_prep_value(value)
            return connection.ops.adapt_datefield_value(value)

        def value_to_string(self, obj):
            val = self.value_from_object(obj)
            return '' if val is None else val.isoformat()

        def formfield(self, **kwargs):
            defaults = {'form_class': forms.DateField}
            defaults.update(kwargs)
            return super(DateField, self).formfield(**defaults)

    class DateTimeField(DateField):
        empty_strings_allowed = False
        default_error_messages = {
            'invalid': _("'%(value)s' value has an invalid format. It must be in "
                         "YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ] format."),
            'invalid_date': _("'%(value)s' value has the correct format "
                              "(YYYY-MM-DD) but it is an invalid date."),
            'invalid_datetime': _("'%(value)s' value has the correct format "
                                  "(YYYY-MM-DD HH:MM[:ss[.uuuuuu]][TZ]) "
                                  "but it is an invalid date/time."),
        }
        description = _("Date (with time)")

        # __init__ is inherited from DateField

        def _check_fix_default_value(self):
            """
            Adds a warning to the checks framework stating, that using an actual
            date or datetime value is probably wrong; it's only being evaluated on
            server start-up.

            For details see ticket #21905
            """
            if not self.has_default():
                return []

            now = timezone.now()
            if not timezone.is_naive(now):
                now = timezone.make_naive(now, timezone.utc)
            value = self.default
            if isinstance(value, datetime.datetime):
                second_offset = datetime.timedelta(seconds=10)
                lower = now - second_offset
                upper = now + second_offset
                if timezone.is_aware(value):
                    value = timezone.make_naive(value, timezone.utc)
            elif isinstance(value, datetime.date):
                second_offset = datetime.timedelta(seconds=10)
                lower = now - second_offset
                lower = datetime.datetime(lower.year, lower.month, lower.day)
                upper = now + second_offset
                upper = datetime.datetime(upper.year, upper.month, upper.day)
                value = datetime.datetime(value.year, value.month, value.day)
            else:
                # No explicit date / datetime value -- no checks necessary
                return []
            if lower <= value <= upper:
                return [
                    checks.Warning(
                        'Fixed default value provided.',
                        hint='It seems you set a fixed date / time / datetime '
                             'value as default for this field. This may not be '
                             'what you want. If you want to have the current date '
                             'as default, use `django.utils.timezone.now`',
                        obj=self,
                        id='fields.W161',
                    )
                ]

            return []

        def get_internal_type(self):
            return "DateTimeField"

        def to_python(self, value):
            if value is None:
                return value
            if isinstance(value, datetime.datetime):
                return value
            if isinstance(value, datetime.date):
                value = datetime.datetime(value.year, value.month, value.day)
                if settings.USE_TZ:
                    # For backwards compatibility, interpret naive datetimes in
                    # local time. This won't work during DST change, but we can't
                    # do much about it, so we let the exceptions percolate up the
                    # call stack.
                    warnings.warn("DateTimeField %s.%s received a naive datetime "
                                  "(%s) while time zone support is active." %
                                  (self.model.__name__, self.name, value),
                                  RuntimeWarning)
                    default_timezone = timezone.get_default_timezone()
                    value = timezone.make_aware(value, default_timezone)
                return value

            try:
                parsed = parse_datetime(value)
                if parsed is not None:
                    return parsed
            except ValueError:
                raise exceptions.ValidationError(
                    self.error_messages['invalid_datetime'],
                    code='invalid_datetime',
                    params={'value': value},
                )

            try:
                parsed = parse_date(value)
                if parsed is not None:
                    return datetime.datetime(parsed.year, parsed.month, parsed.day)
            except ValueError:
                raise exceptions.ValidationError(
                    self.error_messages['invalid_date'],
                    code='invalid_date',
                    params={'value': value},
                )

            raise exceptions.ValidationError(
                self.error_messages['invalid'],
                code='invalid',
                params={'value': value},
            )

        def pre_save(self, model_instance, add):
            if self.auto_now or (self.auto_now_add and add):
                value = timezone.now()
                setattr(model_instance, self.attname, value)
                return value
            else:
                return super(DateTimeField, self).pre_save(model_instance, add)

        # contribute_to_class is inherited from DateField, it registers
        # get_next_by_FOO and get_prev_by_FOO

        def get_prep_value(self, value):
            value = super(DateTimeField, self).get_prep_value(value)
            value = self.to_python(value)
            if value is not None and settings.USE_TZ and timezone.is_naive(value):
                # For backwards compatibility, interpret naive datetimes in local
                # time. This won't work during DST change, but we can't do much
                # about it, so we let the exceptions percolate up the call stack.
                try:
                    name = '%s.%s' % (self.model.__name__, self.name)
                except AttributeError:
                    name = '(unbound)'
                warnings.warn("DateTimeField %s received a naive datetime (%s)"
                              " while time zone support is active." %
                              (name, value),
                              RuntimeWarning)
                default_timezone = timezone.get_default_timezone()
                value = timezone.make_aware(value, default_timezone)
            return value

        def get_db_prep_value(self, value, connection, prepared=False):
            # Casts datetimes into the format expected by the backend
            if not prepared:
                value = self.get_prep_value(value)
            return connection.ops.adapt_datetimefield_value(value)

        def value_to_string(self, obj):
            val = self.value_from_object(obj)
            return '' if val is None else val.isoformat()

        def formfield(self, **kwargs):
            defaults = {'form_class': forms.DateTimeField}
            defaults.update(kwargs)
            return super(DateTimeField, self).formfield(**defaults)

    class DecimalField(Field):
        empty_strings_allowed = False
        default_error_messages = {
            'invalid': _("'%(value)s' value must be a decimal number."),
        }
        description = _("Decimal number")

        def __init__(self, verbose_name=None, name=None, max_digits=None,
                     decimal_places=None, **kwargs):
            self.max_digits, self.decimal_places = max_digits, decimal_places
            super(DecimalField, self).__init__(verbose_name, name, **kwargs)

        def check(self, **kwargs):
            errors = super(DecimalField, self).check(**kwargs)

            digits_errors = self._check_decimal_places()
            digits_errors.extend(self._check_max_digits())
            if not digits_errors:
                errors.extend(self._check_decimal_places_and_max_digits(**kwargs))
            else:
                errors.extend(digits_errors)
            return errors

        def _check_decimal_places(self):
            try:
                decimal_places = int(self.decimal_places)
                if decimal_places < 0:
                    raise ValueError()
            except TypeError:
                return [
                    checks.Error(
                        "DecimalFields must define a 'decimal_places' attribute.",
                        obj=self,
                        id='fields.E130',
                    )
                ]
            except ValueError:
                return [
                    checks.Error(
                        "'decimal_places' must be a non-negative integer.",
                        obj=self,
                        id='fields.E131',
                    )
                ]
            else:
                return []

        def _check_max_digits(self):
            try:
                max_digits = int(self.max_digits)
                if max_digits <= 0:
                    raise ValueError()
            except TypeError:
                return [
                    checks.Error(
                        "DecimalFields must define a 'max_digits' attribute.",
                        obj=self,
                        id='fields.E132',
                    )
                ]
            except ValueError:
                return [
                    checks.Error(
                        "'max_digits' must be a positive integer.",
                        obj=self,
                        id='fields.E133',
                    )
                ]
            else:
                return []

        def _check_decimal_places_and_max_digits(self, **kwargs):
            if int(self.decimal_places) > int(self.max_digits):
                return [
                    checks.Error(
                        "'max_digits' must be greater or equal to 'decimal_places'.",
                        obj=self,
                        id='fields.E134',
                    )
                ]
            return []

        @cached_property
        def validators(self):
            return super(DecimalField, self).validators + [
                validators.DecimalValidator(self.max_digits, self.decimal_places)
            ]

        def deconstruct(self):
            name, path, args, kwargs = super(DecimalField, self).deconstruct()
            if self.max_digits is not None:
                kwargs['max_digits'] = self.max_digits
            if self.decimal_places is not None:
                kwargs['decimal_places'] = self.decimal_places
            return name, path, args, kwargs

        def get_internal_type(self):
            return "DecimalField"

        def to_python(self, value):
            if value is None:
                return value
            try:
                return decimal.Decimal(value)
            except decimal.InvalidOperation:
                raise exceptions.ValidationError(
                    self.error_messages['invalid'],
                    code='invalid',
                    params={'value': value},
                )

        def _format(self, value):
            if isinstance(value, six.string_types):
                return value
            else:
                return self.format_number(value)

        def format_number(self, value):
            """
            Formats a number into a string with the requisite number of digits and
            decimal places.
            """
            # Method moved to django.db.backends.utils.
            #
            # It is preserved because it is used by the oracle backend
            # (django.db.backends.oracle.query), and also for
            # backwards-compatibility with any external code which may have used
            # this method.
            from django.db.backends import utils
            return utils.format_number(value, self.max_digits, self.decimal_places)

        def get_db_prep_save(self, value, connection):
            return connection.ops.adapt_decimalfield_value(self.to_python(value), self.max_digits, self.decimal_places)

        def get_prep_value(self, value):
            value = super(DecimalField, self).get_prep_value(value)
            return self.to_python(value)

        def formfield(self, **kwargs):
            defaults = {
                'max_digits': self.max_digits,
                'decimal_places': self.decimal_places,
                'form_class': forms.DecimalField,
            }
            defaults.update(kwargs)
            return super(DecimalField, self).formfield(**defaults)

    class DurationField(Field):
        """Stores timedelta objects.

        Uses interval on postgres, INVERAL DAY TO SECOND on Oracle, and bigint of
        microseconds on other databases.
        """
        empty_strings_allowed = False
        default_error_messages = {
            'invalid': _("'%(value)s' value has an invalid format. It must be in "
                         "[DD] [HH:[MM:]]ss[.uuuuuu] format.")
        }
        description = _("Duration")

        def get_internal_type(self):
            return "DurationField"

        def to_python(self, value):
            if value is None:
                return value
            if isinstance(value, datetime.timedelta):
                return value
            try:
                parsed = parse_duration(value)
            except ValueError:
                pass
            else:
                if parsed is not None:
                    return parsed

            raise exceptions.ValidationError(
                self.error_messages['invalid'],
                code='invalid',
                params={'value': value},
            )

        def get_db_prep_value(self, value, connection, prepared=False):
            if connection.features.has_native_duration_field:
                return value
            if value is None:
                return None
            # Discard any fractional microseconds due to floating point arithmetic.
            return int(round(value.total_seconds() * 1000000))

        def get_db_converters(self, connection):
            converters = []
            if not connection.features.has_native_duration_field:
                converters.append(connection.ops.convert_durationfield_value)
            return converters + super(DurationField, self).get_db_converters(connection)

        def value_to_string(self, obj):
            val = self.value_from_object(obj)
            return '' if val is None else duration_string(val)

        def formfield(self, **kwargs):
            defaults = {
                'form_class': forms.DurationField,
            }
            defaults.update(kwargs)
            return super(DurationField, self).formfield(**defaults)

    class EmailField(CharField):
        default_validators = [validators.validate_email]
        description = _("Email address")

        def __init__(self, *args, **kwargs):
            # max_length=254 to be compliant with RFCs 3696 and 5321
            kwargs['max_length'] = kwargs.get('max_length', 254)
            super(EmailField, self).__init__(*args, **kwargs)

        def deconstruct(self):
            name, path, args, kwargs = super(EmailField, self).deconstruct()
            # We do not exclude max_length if it matches default as we want to change
            # the default in future.
            return name, path, args, kwargs

        def formfield(self, **kwargs):
            # As with CharField, this will cause email validation to be performed
            # twice.
            defaults = {
                'form_class': forms.EmailField,
            }
            defaults.update(kwargs)
            return super(EmailField, self).formfield(**defaults)

    class FilePathField(Field):
        description = _("File path")

        def __init__(self, verbose_name=None, name=None, path='', match=None,
                     recursive=False, allow_files=True, allow_folders=False, **kwargs):
            self.path, self.match, self.recursive = path, match, recursive
            self.allow_files, self.allow_folders = allow_files, allow_folders
            kwargs['max_length'] = kwargs.get('max_length', 100)
            super(FilePathField, self).__init__(verbose_name, name, **kwargs)

        def check(self, **kwargs):
            errors = super(FilePathField, self).check(**kwargs)
            errors.extend(self._check_allowing_files_or_folders(**kwargs))
            return errors

        def _check_allowing_files_or_folders(self, **kwargs):
            if not self.allow_files and not self.allow_folders:
                return [
                    checks.Error(
                        "FilePathFields must have either 'allow_files' or 'allow_folders' set to True.",
                        obj=self,
                        id='fields.E140',
                    )
                ]
            return []

        def deconstruct(self):
            name, path, args, kwargs = super(FilePathField, self).deconstruct()
            if self.path != '':
                kwargs['path'] = self.path
            if self.match is not None:
                kwargs['match'] = self.match
            if self.recursive is not False:
                kwargs['recursive'] = self.recursive
            if self.allow_files is not True:
                kwargs['allow_files'] = self.allow_files
            if self.allow_folders is not False:
                kwargs['allow_folders'] = self.allow_folders
            if kwargs.get("max_length") == 100:
                del kwargs["max_length"]
            return name, path, args, kwargs

        def get_prep_value(self, value):
            value = super(FilePathField, self).get_prep_value(value)
            if value is None:
                return None
            return six.text_type(value)

        def formfield(self, **kwargs):
            defaults = {
                'path': self.path,
                'match': self.match,
                'recursive': self.recursive,
                'form_class': forms.FilePathField,
                'allow_files': self.allow_files,
                'allow_folders': self.allow_folders,
            }
            defaults.update(kwargs)
            return super(FilePathField, self).formfield(**defaults)

        def get_internal_type(self):
            return "FilePathField"

    class FloatField(Field):
        empty_strings_allowed = False
        default_error_messages = {
            'invalid': _("'%(value)s' value must be a float."),
        }
        description = _("Floating point number")

        def get_prep_value(self, value):
            value = super(FloatField, self).get_prep_value(value)
            if value is None:
                return None
            return float(value)

        def get_internal_type(self):
            return "FloatField"

        def to_python(self, value):
            if value is None:
                return value
            try:
                return float(value)
            except (TypeError, ValueError):
                raise exceptions.ValidationError(
                    self.error_messages['invalid'],
                    code='invalid',
                    params={'value': value},
                )

        def formfield(self, **kwargs):
            defaults = {'form_class': forms.FloatField}
            defaults.update(kwargs)
            return super(FloatField, self).formfield(**defaults)

    class IntegerField(Field):
        empty_strings_allowed = False
        default_error_messages = {
            'invalid': _("'%(value)s' value must be an integer."),
        }
        description = _("Integer")

        def check(self, **kwargs):
            errors = super(IntegerField, self).check(**kwargs)
            errors.extend(self._check_max_length_warning())
            return errors

        def _check_max_length_warning(self):
            if self.max_length is not None:
                return [
                    checks.Warning(
                        "'max_length' is ignored when used with IntegerField",
                        hint="Remove 'max_length' from field",
                        obj=self,
                        id='fields.W122',
                    )
                ]
            return []

        @cached_property
        def validators(self):
            # These validators can't be added at field initialization time since
            # they're based on values retrieved from `connection`.
            validators_ = super(IntegerField, self).validators
            internal_type = self.get_internal_type()
            min_value, max_value = connection.ops.integer_field_range(internal_type)
            if min_value is not None:
                for validator in validators_:
                    if isinstance(validator, validators.MinValueValidator) and validator.limit_value >= min_value:
                        break
                else:
                    validators_.append(validators.MinValueValidator(min_value))
            if max_value is not None:
                for validator in validators_:
                    if isinstance(validator, validators.MaxValueValidator) and validator.limit_value <= max_value:
                        break
                else:
                    validators_.append(validators.MaxValueValidator(max_value))
            return validators_

        def get_prep_value(self, value):
            value = super(IntegerField, self).get_prep_value(value)
            if value is None:
                return None
            return int(value)

        def get_internal_type(self):
            return "IntegerField"

        def to_python(self, value):
            if value is None:
                return value
            try:
                return int(value)
            except (TypeError, ValueError):
                raise exceptions.ValidationError(
                    self.error_messages['invalid'],
                    code='invalid',
                    params={'value': value},
                )

        def formfield(self, **kwargs):
            defaults = {'form_class': forms.IntegerField}
            defaults.update(kwargs)
            return super(IntegerField, self).formfield(**defaults)

    class BigIntegerField(IntegerField):
        empty_strings_allowed = False
        description = _("Big (8 byte) integer")
        MAX_BIGINT = 9223372036854775807

        def get_internal_type(self):
            return "BigIntegerField"

        def formfield(self, **kwargs):
            defaults = {'min_value': -BigIntegerField.MAX_BIGINT - 1,
                        'max_value': BigIntegerField.MAX_BIGINT}
            defaults.update(kwargs)
            return super(BigIntegerField, self).formfield(**defaults)

    class IPAddressField(Field):
        empty_strings_allowed = False
        description = _("IPv4 address")
        system_check_removed_details = {
            'msg': (
                'IPAddressField has been removed except for support in '
                'historical migrations.'
            ),
            'hint': 'Use GenericIPAddressField instead.',
            'id': 'fields.E900',
        }

        def __init__(self, *args, **kwargs):
            kwargs['max_length'] = 15
            super(IPAddressField, self).__init__(*args, **kwargs)

        def deconstruct(self):
            name, path, args, kwargs = super(IPAddressField, self).deconstruct()
            del kwargs['max_length']
            return name, path, args, kwargs

        def get_prep_value(self, value):
            value = super(IPAddressField, self).get_prep_value(value)
            if value is None:
                return None
            return six.text_type(value)

        def get_internal_type(self):
            return "IPAddressField"

    class GenericIPAddressField(Field):
        empty_strings_allowed = False
        description = _("IP address")
        default_error_messages = {}

        def __init__(self, verbose_name=None, name=None, protocol='both',
                     unpack_ipv4=False, *args, **kwargs):
            self.unpack_ipv4 = unpack_ipv4
            self.protocol = protocol
            self.default_validators, invalid_error_message = \
                validators.ip_address_validators(protocol, unpack_ipv4)
            self.default_error_messages['invalid'] = invalid_error_message
            kwargs['max_length'] = 39
            super(GenericIPAddressField, self).__init__(verbose_name, name, *args,
                                                        **kwargs)

        def check(self, **kwargs):
            errors = super(GenericIPAddressField, self).check(**kwargs)
            errors.extend(self._check_blank_and_null_values(**kwargs))
            return errors

        def _check_blank_and_null_values(self, **kwargs):
            if not getattr(self, 'null', False) and getattr(self, 'blank', False):
                return [
                    checks.Error(
                        'GenericIPAddressFields cannot have blank=True if null=False, '
                        'as blank values are stored as nulls.',
                        obj=self,
                        id='fields.E150',
                    )
                ]
            return []

        def deconstruct(self):
            name, path, args, kwargs = super(GenericIPAddressField, self).deconstruct()
            if self.unpack_ipv4 is not False:
                kwargs['unpack_ipv4'] = self.unpack_ipv4
            if self.protocol != "both":
                kwargs['protocol'] = self.protocol
            if kwargs.get("max_length") == 39:
                del kwargs['max_length']
            return name, path, args, kwargs

        def get_internal_type(self):
            return "GenericIPAddressField"

        def to_python(self, value):
            if value is None:
                return None
            if not isinstance(value, six.string_types):
                value = force_text(value)
            value = value.strip()
            if ':' in value:
                return clean_ipv6_address(value, self.unpack_ipv4, self.error_messages['invalid'])
            return value

        def get_db_prep_value(self, value, connection, prepared=False):
            if not prepared:
                value = self.get_prep_value(value)
            return connection.ops.adapt_ipaddressfield_value(value)

        def get_prep_value(self, value):
            value = super(GenericIPAddressField, self).get_prep_value(value)
            if value is None:
                return None
            if value and ':' in value:
                try:
                    return clean_ipv6_address(value, self.unpack_ipv4)
                except exceptions.ValidationError:
                    pass
            return six.text_type(value)

        def formfield(self, **kwargs):
            defaults = {
                'protocol': self.protocol,
                'form_class': forms.GenericIPAddressField,
            }
            defaults.update(kwargs)
            return super(GenericIPAddressField, self).formfield(**defaults)

    class NullBooleanField(Field):
        empty_strings_allowed = False
        default_error_messages = {
            'invalid': _("'%(value)s' value must be either None, True or False."),
        }
        description = _("Boolean (Either True, False or None)")

        def __init__(self, *args, **kwargs):
            kwargs['null'] = True
            kwargs['blank'] = True
            super(NullBooleanField, self).__init__(*args, **kwargs)

        def deconstruct(self):
            name, path, args, kwargs = super(NullBooleanField, self).deconstruct()
            del kwargs['null']
            del kwargs['blank']
            return name, path, args, kwargs

        def get_internal_type(self):
            return "NullBooleanField"

        def to_python(self, value):
            if value is None:
                return None
            if value in (True, False):
                return bool(value)
            if value in ('None',):
                return None
            if value in ('t', 'True', '1'):
                return True
            if value in ('f', 'False', '0'):
                return False
            raise exceptions.ValidationError(
                self.error_messages['invalid'],
                code='invalid',
                params={'value': value},
            )

        def get_prep_value(self, value):
            value = super(NullBooleanField, self).get_prep_value(value)
            if value is None:
                return None
            return self.to_python(value)

        def formfield(self, **kwargs):
            defaults = {
                'form_class': forms.NullBooleanField,
                'required': not self.blank,
                'label': capfirst(self.verbose_name),
                'help_text': self.help_text}
            defaults.update(kwargs)
            return super(NullBooleanField, self).formfield(**defaults)

    class PositiveIntegerRelDbTypeMixin(object):

        def rel_db_type(self, connection):
            """
            Return the data type that a related field pointing to this field should
            use. In most cases, a foreign key pointing to a positive integer
            primary key will have an integer column data type but some databases
            (e.g. MySQL) have an unsigned integer type. In that case
            (related_fields_match_type=True), the primary key should return its
            db_type.
            """
            if connection.features.related_fields_match_type:
                return self.db_type(connection)
            else:
                return IntegerField().db_type(connection=connection)

    class PositiveIntegerField(PositiveIntegerRelDbTypeMixin, IntegerField):
        description = _("Positive integer")

        def get_internal_type(self):
            return "PositiveIntegerField"

        def formfield(self, **kwargs):
            defaults = {'min_value': 0}
            defaults.update(kwargs)
            return super(PositiveIntegerField, self).formfield(**defaults)

    class PositiveSmallIntegerField(PositiveIntegerRelDbTypeMixin, IntegerField):
        description = _("Positive small integer")

        def get_internal_type(self):
            return "PositiveSmallIntegerField"

        def formfield(self, **kwargs):
            defaults = {'min_value': 0}
            defaults.update(kwargs)
            return super(PositiveSmallIntegerField, self).formfield(**defaults)

    class SlugField(CharField):
        default_validators = [validators.validate_slug]
        description = _("Slug (up to %(max_length)s)")

        def __init__(self, *args, **kwargs):
            kwargs['max_length'] = kwargs.get('max_length', 50)
            # Set db_index=True unless it's been set manually.
            if 'db_index' not in kwargs:
                kwargs['db_index'] = True
            self.allow_unicode = kwargs.pop('allow_unicode', False)
            if self.allow_unicode:
                self.default_validators = [validators.validate_unicode_slug]
            super(SlugField, self).__init__(*args, **kwargs)

        def deconstruct(self):
            name, path, args, kwargs = super(SlugField, self).deconstruct()
            if kwargs.get("max_length") == 50:
                del kwargs['max_length']
            if self.db_index is False:
                kwargs['db_index'] = False
            else:
                del kwargs['db_index']
            if self.allow_unicode is not False:
                kwargs['allow_unicode'] = self.allow_unicode
            return name, path, args, kwargs

        def get_internal_type(self):
            return "SlugField"

        def formfield(self, **kwargs):
            defaults = {'form_class': forms.SlugField, 'allow_unicode': self.allow_unicode}
            defaults.update(kwargs)
            return super(SlugField, self).formfield(**defaults)

    class SmallIntegerField(IntegerField):
        description = _("Small integer")

        def get_internal_type(self):
            return "SmallIntegerField"

    class TextField(Field):
        description = _("Text")

        def get_internal_type(self):
            return "TextField"

        def to_python(self, value):
            if isinstance(value, six.string_types) or value is None:
                return value
            return smart_text(value)

        def get_prep_value(self, value):
            value = super(TextField, self).get_prep_value(value)
            return self.to_python(value)

        def formfield(self, **kwargs):
            # Passing max_length to forms.CharField means that the value's length
            # will be validated twice. This is considered acceptable since we want
            # the value in the form field (to pass into widget for example).
            defaults = {'max_length': self.max_length, 'widget': forms.Textarea}
            defaults.update(kwargs)
            return super(TextField, self).formfield(**defaults)

    class TimeField(DateTimeCheckMixin, Field):
        empty_strings_allowed = False
        default_error_messages = {
            'invalid': _("'%(value)s' value has an invalid format. It must be in "
                         "HH:MM[:ss[.uuuuuu]] format."),
            'invalid_time': _("'%(value)s' value has the correct format "
                              "(HH:MM[:ss[.uuuuuu]]) but it is an invalid time."),
        }
        description = _("Time")

        def __init__(self, verbose_name=None, name=None, auto_now=False,
                     auto_now_add=False, **kwargs):
            self.auto_now, self.auto_now_add = auto_now, auto_now_add
            if auto_now or auto_now_add:
                kwargs['editable'] = False
                kwargs['blank'] = True
            super(TimeField, self).__init__(verbose_name, name, **kwargs)

        def _check_fix_default_value(self):
            """
            Adds a warning to the checks framework stating, that using an actual
            time or datetime value is probably wrong; it's only being evaluated on
            server start-up.

            For details see ticket #21905
            """
            if not self.has_default():
                return []

            now = timezone.now()
            if not timezone.is_naive(now):
                now = timezone.make_naive(now, timezone.utc)
            value = self.default
            if isinstance(value, datetime.datetime):
                second_offset = datetime.timedelta(seconds=10)
                lower = now - second_offset
                upper = now + second_offset
                if timezone.is_aware(value):
                    value = timezone.make_naive(value, timezone.utc)
            elif isinstance(value, datetime.time):
                second_offset = datetime.timedelta(seconds=10)
                lower = now - second_offset
                upper = now + second_offset
                value = datetime.datetime.combine(now.date(), value)
                if timezone.is_aware(value):
                    value = timezone.make_naive(value, timezone.utc).time()
            else:
                # No explicit time / datetime value -- no checks necessary
                return []
            if lower <= value <= upper:
                return [
                    checks.Warning(
                        'Fixed default value provided.',
                        hint='It seems you set a fixed date / time / datetime '
                             'value as default for this field. This may not be '
                             'what you want. If you want to have the current date '
                             'as default, use `django.utils.timezone.now`',
                        obj=self,
                        id='fields.W161',
                    )
                ]

            return []

        def deconstruct(self):
            name, path, args, kwargs = super(TimeField, self).deconstruct()
            if self.auto_now is not False:
                kwargs["auto_now"] = self.auto_now
            if self.auto_now_add is not False:
                kwargs["auto_now_add"] = self.auto_now_add
            if self.auto_now or self.auto_now_add:
                del kwargs['blank']
                del kwargs['editable']
            return name, path, args, kwargs

        def get_internal_type(self):
            return "TimeField"

        def to_python(self, value):
            if value is None:
                return None
            if isinstance(value, datetime.time):
                return value
            if isinstance(value, datetime.datetime):
                # Not usually a good idea to pass in a datetime here (it loses
                # information), but this can be a side-effect of interacting with a
                # database backend (e.g. Oracle), so we'll be accommodating.
                return value.time()

            try:
                parsed = parse_time(value)
                if parsed is not None:
                    return parsed
            except ValueError:
                raise exceptions.ValidationError(
                    self.error_messages['invalid_time'],
                    code='invalid_time',
                    params={'value': value},
                )

            raise exceptions.ValidationError(
                self.error_messages['invalid'],
                code='invalid',
                params={'value': value},
            )

        def pre_save(self, model_instance, add):
            if self.auto_now or (self.auto_now_add and add):
                value = datetime.datetime.now().time()
                setattr(model_instance, self.attname, value)
                return value
            else:
                return super(TimeField, self).pre_save(model_instance, add)

        def get_prep_value(self, value):
            value = super(TimeField, self).get_prep_value(value)
            return self.to_python(value)

        def get_db_prep_value(self, value, connection, prepared=False):
            # Casts times into the format expected by the backend
            if not prepared:
                value = self.get_prep_value(value)
            return connection.ops.adapt_timefield_value(value)

        def value_to_string(self, obj):
            val = self.value_from_object(obj)
            return '' if val is None else val.isoformat()

        def formfield(self, **kwargs):
            defaults = {'form_class': forms.TimeField}
            defaults.update(kwargs)
            return super(TimeField, self).formfield(**defaults)

    class URLField(CharField):
        default_validators = [validators.URLValidator()]
        description = _("URL")

        def __init__(self, verbose_name=None, name=None, **kwargs):
            kwargs['max_length'] = kwargs.get('max_length', 200)
            super(URLField, self).__init__(verbose_name, name, **kwargs)

        def deconstruct(self):
            name, path, args, kwargs = super(URLField, self).deconstruct()
            if kwargs.get("max_length") == 200:
                del kwargs['max_length']
            return name, path, args, kwargs

        def formfield(self, **kwargs):
            # As with CharField, this will cause URL validation to be performed
            # twice.
            defaults = {
                'form_class': forms.URLField,
            }
            defaults.update(kwargs)
            return super(URLField, self).formfield(**defaults)

    class BinaryField(Field):
        description = _("Raw binary data")
        empty_values = [None, b'']

        def __init__(self, *args, **kwargs):
            kwargs['editable'] = False
            super(BinaryField, self).__init__(*args, **kwargs)
            if self.max_length is not None:
                self.validators.append(validators.MaxLengthValidator(self.max_length))

        def deconstruct(self):
            name, path, args, kwargs = super(BinaryField, self).deconstruct()
            del kwargs['editable']
            return name, path, args, kwargs

        def get_internal_type(self):
            return "BinaryField"

        def get_placeholder(self, value, compiler, connection):
            return connection.ops.binary_placeholder_sql(value)

        def get_default(self):
            if self.has_default() and not callable(self.default):
                return self.default
            default = super(BinaryField, self).get_default()
            if default == '':
                return b''
            return default

        def get_db_prep_value(self, value, connection, prepared=False):
            value = super(BinaryField, self).get_db_prep_value(value, connection, prepared)
            if value is not None:
                return connection.Database.Binary(value)
            return value

        def value_to_string(self, obj):
            """Binary data is serialized as base64"""
            return b64encode(force_bytes(self.value_from_object(obj))).decode('ascii')

        def to_python(self, value):
            # If it's a string, it should be base64-encoded data
            if isinstance(value, six.text_type):
                return six.memoryview(b64decode(force_bytes(value)))
            return value

    class UUIDField(Field):
        default_error_messages = {
            'invalid': _("'%(value)s' is not a valid UUID."),
        }
        description = 'Universally unique identifier'
        empty_strings_allowed = False

        def __init__(self, verbose_name=None, **kwargs):
            kwargs['max_length'] = 32
            super(UUIDField, self).__init__(verbose_name, **kwargs)

        def deconstruct(self):
            name, path, args, kwargs = super(UUIDField, self).deconstruct()
            del kwargs['max_length']
            return name, path, args, kwargs

        def get_internal_type(self):
            return "UUIDField"

        def get_db_prep_value(self, value, connection, prepared=False):
            if value is None:
                return None
            if not isinstance(value, uuid.UUID):
                try:
                    value = uuid.UUID(value)
                except AttributeError:
                    raise TypeError(self.error_messages['invalid'] % {'value': value})

            if connection.features.has_native_uuid_field:
                return value
            return value.hex

        def to_python(self, value):
            if value and not isinstance(value, uuid.UUID):
                try:
                    return uuid.UUID(value)
                except ValueError:
                    raise exceptions.ValidationError(
                        self.error_messages['invalid'],
                        code='invalid',
                        params={'value': value},
                    )
            return value

        def formfield(self, **kwargs):
            defaults = {
                'form_class': forms.UUIDField,
            }
            defaults.update(kwargs)
            return super(UUIDField, self).formfield(**defaults)