# documentación de Django - 1.10.7

# Código fuente para django.db.models.fields.files

    import datetime
    import os
    import posixpath
    import warnings

    from django import forms
    from django.core import checks
    from django.core.files.base import File
    from django.core.files.images import ImageFile
    from django.core.files.storage import default_storage
    from django.db.models import signals
    from django.db.models.fields import Field
    from django.utils import six
    from django.utils.deprecation import RemovedInDjango20Warning
    from django.utils.encoding import force_str, force_text
    from django.utils.translation import ugettext_lazy as _

    class FieldFile(File):
        def __init__(self, instance, field, name):
            super(FieldFile, self).__init__(None, name)
            self.instance = instance
            self.field = field
            self.storage = field.storage
            self._committed = True

        def __eq__(self, other):
            # Older code may be expecting FileField values to be simple strings.
            # By overriding the == operator, it can remain backwards compatibility.
            if hasattr(other, 'name'):
                return self.name == other.name
            return self.name == other

        def __ne__(self, other):
            return not self.__eq__(other)

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

        # The standard File contains most of the necessary properties, but
        # FieldFiles can be instantiated without a name, so that needs to
        # be checked for here.

        def _require_file(self):
            if not self:
                raise ValueError("The '%s' attribute has no file associated with it." % self.field.name)

        def _get_file(self):
            self._require_file()
            if not hasattr(self, '_file') or self._file is None:
                self._file = self.storage.open(self.name, 'rb')
            return self._file

        def _set_file(self, file):
            self._file = file

        def _del_file(self):
            del self._file

        file = property(_get_file, _set_file, _del_file)

        def _get_path(self):
            self._require_file()
            return self.storage.path(self.name)
        path = property(_get_path)

        def _get_url(self):
            self._require_file()
            return self.storage.url(self.name)
        url = property(_get_url)

        def _get_size(self):
            self._require_file()
            if not self._committed:
                return self.file.size
            return self.storage.size(self.name)
        size = property(_get_size)

        def open(self, mode='rb'):
            self._require_file()
            self.file.open(mode)
        # open() doesn't alter the file's contents, but it does reset the pointer
        open.alters_data = True

        # In addition to the standard File API, FieldFiles have extra methods
        # to further manipulate the underlying file, as well as update the
        # associated model instance.

        def save(self, name, content, save=True):
            name = self.field.generate_filename(self.instance, name)
            self.name = self.storage.save(name, content, max_length=self.field.max_length)
            setattr(self.instance, self.field.name, self.name)
            self._committed = True

            # Save the object because it has changed, unless save is False
            if save:
                self.instance.save()
        save.alters_data = True

        def delete(self, save=True):
            if not self:
                return
            # Only close the file if it's already open, which we know by the
            # presence of self._file
            if hasattr(self, '_file'):
                self.close()
                del self.file

            self.storage.delete(self.name)

            self.name = None
            setattr(self.instance, self.field.name, self.name)
            self._committed = False

            if save:
                self.instance.save()
        delete.alters_data = True

        def _get_closed(self):
            file = getattr(self, '_file', None)
            return file is None or file.closed
        closed = property(_get_closed)

        def close(self):
            file = getattr(self, '_file', None)
            if file is not None:
                file.close()

        def __getstate__(self):
            # FieldFile needs access to its associated model field and an instance
            # it's attached to in order to work properly, but the only necessary
            # data to be pickled is the file's name itself. Everything else will
            # be restored later, by FileDescriptor below.
            return {'name': self.name, 'closed': False, '_committed': True, '_file': None}

    class FileDescriptor(object):
        """
        The descriptor for the file attribute on the model instance. Returns a
        FieldFile when accessed so you can do stuff like::

            >>> from myapp.models import MyModel
            >>> instance = MyModel.objects.get(pk=1)
            >>> instance.file.size

        Assigns a file object on assignment so you can do::

            >>> with open('/path/to/hello.world', 'r') as f:
            ...     instance.file = File(f)
        """
        def __init__(self, field):
            self.field = field

        def __get__(self, instance, cls=None):
            if instance is None:
                return self

            # This is slightly complicated, so worth an explanation.
            # instance.file`needs to ultimately return some instance of `File`,
            # probably a subclass. Additionally, this returned object needs to have
            # the FieldFile API so that users can easily do things like
            # instance.file.path and have that delegated to the file storage engine.
            # Easy enough if we're strict about assignment in __set__, but if you
            # peek below you can see that we're not. So depending on the current
            # value of the field we have to dynamically construct some sort of
            # "thing" to return.

            # The instance dict contains whatever was originally assigned
            # in __set__.
            if self.field.name in instance.__dict__:
                file = instance.__dict__[self.field.name]
            else:
                instance.refresh_from_db(fields=[self.field.name])
                file = getattr(instance, self.field.name)

            # If this value is a string (instance.file = "path/to/file") or None
            # then we simply wrap it with the appropriate attribute class according
            # to the file field. [This is FieldFile for FileFields and
            # ImageFieldFile for ImageFields; it's also conceivable that user
            # subclasses might also want to subclass the attribute class]. This
            # object understands how to convert a path to a file, and also how to
            # handle None.
            if isinstance(file, six.string_types) or file is None:
                attr = self.field.attr_class(instance, self.field, file)
                instance.__dict__[self.field.name] = attr

            # Other types of files may be assigned as well, but they need to have
            # the FieldFile interface added to them. Thus, we wrap any other type of
            # File inside a FieldFile (well, the field's attr_class, which is
            # usually FieldFile).
            elif isinstance(file, File) and not isinstance(file, FieldFile):
                file_copy = self.field.attr_class(instance, self.field, file.name)
                file_copy.file = file
                file_copy._committed = False
                instance.__dict__[self.field.name] = file_copy

            # Finally, because of the (some would say boneheaded) way pickle works,
            # the underlying FieldFile might not actually itself have an associated
            # file. So we need to reset the details of the FieldFile in those cases.
            elif isinstance(file, FieldFile) and not hasattr(file, 'field'):
                file.instance = instance
                file.field = self.field
                file.storage = self.field.storage

            # Make sure that the instance is correct.
            elif isinstance(file, FieldFile) and instance is not file.instance:
                file.instance = instance

            # That was fun, wasn't it?
            return instance.__dict__[self.field.name]

        def __set__(self, instance, value):
            instance.__dict__[self.field.name] = value

    class FileField(Field):

        # The class to wrap instance attributes in. Accessing the file object off
        # the instance will always return an instance of attr_class.
        attr_class = FieldFile

        # The descriptor to use for accessing the attribute off of the class.
        descriptor_class = FileDescriptor

        description = _("File")

        def __init__(self, verbose_name=None, name=None, upload_to='', storage=None, **kwargs):
            self._primary_key_set_explicitly = 'primary_key' in kwargs
            self._unique_set_explicitly = 'unique' in kwargs

            self.storage = storage or default_storage
            self.upload_to = upload_to

            kwargs['max_length'] = kwargs.get('max_length', 100)
            super(FileField, self).__init__(verbose_name, name, **kwargs)

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

        def _check_unique(self):
            if self._unique_set_explicitly:
                return [
                    checks.Error(
                        "'unique' is not a valid argument for a %s." % self.__class__.__name__,
                        obj=self,
                        id='fields.E200',
                    )
                ]
            else:
                return []

        def _check_primary_key(self):
            if self._primary_key_set_explicitly:
                return [
                    checks.Error(
                        "'primary_key' is not a valid argument for a %s." % self.__class__.__name__,
                        obj=self,
                        id='fields.E201',
                    )
                ]
            else:
                return []

        def deconstruct(self):
            name, path, args, kwargs = super(FileField, self).deconstruct()
            if kwargs.get("max_length") == 100:
                del kwargs["max_length"]
            kwargs['upload_to'] = self.upload_to
            if self.storage is not default_storage:
                kwargs['storage'] = self.storage
            return name, path, args, kwargs

        def get_internal_type(self):
            return "FileField"

        def get_prep_value(self, value):
            "Returns field's value prepared for saving into a database."
            value = super(FileField, self).get_prep_value(value)
            # Need to convert File objects provided via a form to unicode for database insertion
            if value is None:
                return None
            return six.text_type(value)

        def pre_save(self, model_instance, add):
            "Returns field's value just before saving."
            file = super(FileField, self).pre_save(model_instance, add)
            if file and not file._committed:
                # Commit the file to storage prior to saving the model
                file.save(file.name, file, save=False)
            return file

        def contribute_to_class(self, cls, name, **kwargs):
            super(FileField, self).contribute_to_class(cls, name, **kwargs)
            setattr(cls, self.name, self.descriptor_class(self))

        def get_directory_name(self):
            warnings.warn(
                'FileField now delegates file name and folder processing to the '
                'storage. get_directory_name() will be removed in Django 2.0.',
                RemovedInDjango20Warning, stacklevel=2
            )
            return os.path.normpath(force_text(datetime.datetime.now().strftime(force_str(self.upload_to))))

        def get_filename(self, filename):
            warnings.warn(
                'FileField now delegates file name and folder processing to the '
                'storage. get_filename() will be removed in Django 2.0.',
                RemovedInDjango20Warning, stacklevel=2
            )
            return os.path.normpath(self.storage.get_valid_name(os.path.basename(filename)))

        def generate_filename(self, instance, filename):
            """
            Apply (if callable) or prepend (if a string) upload_to to the filename,
            then delegate further processing of the name to the storage backend.
            Until the storage layer, all file paths are expected to be Unix style
            (with forward slashes).
            """
            if callable(self.upload_to):
                filename = self.upload_to(instance, filename)
            else:
                dirname = force_text(datetime.datetime.now().strftime(force_str(self.upload_to)))
                filename = posixpath.join(dirname, filename)
            return self.storage.generate_filename(filename)

        def save_form_data(self, instance, data):
            # Important: None means "no change", other false value means "clear"
            # This subtle distinction (rather than a more explicit marker) is
            # needed because we need to consume values that are also sane for a
            # regular (non Model-) Form to find in its cleaned_data dictionary.
            if data is not None:
                # This value will be converted to unicode and stored in the
                # database, so leaving False as-is is not acceptable.
                if not data:
                    data = ''
                setattr(instance, self.name, data)

        def formfield(self, **kwargs):
            defaults = {'form_class': forms.FileField, 'max_length': self.max_length}
            # If a file has been provided previously, then the form doesn't require
            # that a new file is provided this time.
            # The code to mark the form field as not required is used by
            # form_for_instance, but can probably be removed once form_for_instance
            # is gone. ModelForm uses a different method to check for an existing file.
            if 'initial' in kwargs:
                defaults['required'] = False
            defaults.update(kwargs)
            return super(FileField, self).formfield(**defaults)

    class ImageFileDescriptor(FileDescriptor):
        """
        Just like the FileDescriptor, but for ImageFields. The only difference is
        assigning the width/height to the width_field/height_field, if appropriate.
        """
        def __set__(self, instance, value):
            previous_file = instance.__dict__.get(self.field.name)
            super(ImageFileDescriptor, self).__set__(instance, value)

            # To prevent recalculating image dimensions when we are instantiating
            # an object from the database (bug #11084), only update dimensions if
            # the field had a value before this assignment.  Since the default
            # value for FileField subclasses is an instance of field.attr_class,
            # previous_file will only be None when we are called from
            # Model.__init__().  The ImageField.update_dimension_fields method
            # hooked up to the post_init signal handles the Model.__init__() cases.
            # Assignment happening outside of Model.__init__() will trigger the
            # update right here.
            if previous_file is not None:
                self.field.update_dimension_fields(instance, force=True)

    class ImageFieldFile(ImageFile, FieldFile):
        def delete(self, save=True):
            # Clear the image dimensions cache
            if hasattr(self, '_dimensions_cache'):
                del self._dimensions_cache
            super(ImageFieldFile, self).delete(save)

    class ImageField(FileField):
        attr_class = ImageFieldFile
        descriptor_class = ImageFileDescriptor
        description = _("Image")

        def __init__(self, verbose_name=None, name=None, width_field=None, height_field=None, **kwargs):
            self.width_field, self.height_field = width_field, height_field
            super(ImageField, self).__init__(verbose_name, name, **kwargs)

        def check(self, **kwargs):
            errors = super(ImageField, self).check(**kwargs)
            errors.extend(self._check_image_library_installed())
            return errors

        def _check_image_library_installed(self):
            try:
                from PIL import Image  # NOQA
            except ImportError:
                return [
                    checks.Error(
                        'Cannot use ImageField because Pillow is not installed.',
                        hint=('Get Pillow at https://pypi.python.org/pypi/Pillow '
                              'or run command "pip install Pillow".'),
                        obj=self,
                        id='fields.E210',
                    )
                ]
            else:
                return []

        def deconstruct(self):
            name, path, args, kwargs = super(ImageField, self).deconstruct()
            if self.width_field:
                kwargs['width_field'] = self.width_field
            if self.height_field:
                kwargs['height_field'] = self.height_field
            return name, path, args, kwargs

        def contribute_to_class(self, cls, name, **kwargs):
            super(ImageField, self).contribute_to_class(cls, name, **kwargs)
            # Attach update_dimension_fields so that dimension fields declared
            # after their corresponding image field don't stay cleared by
            # Model.__init__, see bug #11196.
            # Only run post-initialization dimension update on non-abstract models
            if not cls._meta.abstract:
                signals.post_init.connect(self.update_dimension_fields, sender=cls)

        def update_dimension_fields(self, instance, force=False, *args, **kwargs):
            """
            Updates field's width and height fields, if defined.

            This method is hooked up to model's post_init signal to update
            dimensions after instantiating a model instance.  However, dimensions
            won't be updated if the dimensions fields are already populated.  This
            avoids unnecessary recalculation when loading an object from the
            database.

            Dimensions can be forced to update with force=True, which is how
            ImageFileDescriptor.__set__ calls this method.
            """
            # Nothing to update if the field doesn't have dimension fields or if
            # the field is deferred.
            has_dimension_fields = self.width_field or self.height_field
            if not has_dimension_fields or self.attname not in instance.__dict__:
                return

            # getattr will call the ImageFileDescriptor's __get__ method, which
            # coerces the assigned value into an instance of self.attr_class
            # (ImageFieldFile in this case).
            file = getattr(instance, self.attname)

            # Nothing to update if we have no file and not being forced to update.
            if not file and not force:
                return

            dimension_fields_filled = not(
                (self.width_field and not getattr(instance, self.width_field)) or
                (self.height_field and not getattr(instance, self.height_field))
            )
            # When both dimension fields have values, we are most likely loading
            # data from the database or updating an image field that already had
            # an image stored.  In the first case, we don't want to update the
            # dimension fields because we are already getting their values from the
            # database.  In the second case, we do want to update the dimensions
            # fields and will skip this return because force will be True since we
            # were called from ImageFileDescriptor.__set__.
            if dimension_fields_filled and not force:
                return

            # file should be an instance of ImageFieldFile or should be None.
            if file:
                width = file.width
                height = file.height
            else:
                # No file, so clear dimensions fields.
                width = None
                height = None

            # Update the width and height fields.
            if self.width_field:
                setattr(instance, self.width_field, width)
            if self.height_field:
                setattr(instance, self.height_field, height)

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