# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το django.db.utils

    import os
    import pkgutil
    from importlib import import_module
    from threading import local

    from django.conf import settings
    from django.core.exceptions import ImproperlyConfigured
    from django.utils import six
    from django.utils._os import npath, upath
    from django.utils.functional import cached_property
    from django.utils.module_loading import import_string

    DEFAULT_DB_ALIAS = 'default'
    DJANGO_VERSION_PICKLE_KEY = '_django_version'

    class Error(Exception if six.PY3 else StandardError):  # NOQA: StandardError undefined on PY3
        pass

    class InterfaceError(Error):
        pass

    class DatabaseError(Error):
        pass

    class DataError(DatabaseError):
        pass

    class OperationalError(DatabaseError):
        pass

    class IntegrityError(DatabaseError):
        pass

    class InternalError(DatabaseError):
        pass

    class ProgrammingError(DatabaseError):
        pass

    class NotSupportedError(DatabaseError):
        pass

    class DatabaseErrorWrapper(object):
        """
        Context manager and decorator that re-throws backend-specific database
        exceptions using Django's common wrappers.
        """

        def __init__(self, wrapper):
            """
            wrapper is a database wrapper.

            It must have a Database attribute defining PEP-249 exceptions.
            """
            self.wrapper = wrapper

        def __enter__(self):
            pass

        def __exit__(self, exc_type, exc_value, traceback):
            if exc_type is None:
                return
            for dj_exc_type in (
                    DataError,
                    OperationalError,
                    IntegrityError,
                    InternalError,
                    ProgrammingError,
                    NotSupportedError,
                    DatabaseError,
                    InterfaceError,
                    Error,
            ):
                db_exc_type = getattr(self.wrapper.Database, dj_exc_type.__name__)
                if issubclass(exc_type, db_exc_type):
                    dj_exc_value = dj_exc_type(*exc_value.args)
                    dj_exc_value.__cause__ = exc_value
                    if not hasattr(exc_value, '__traceback__'):
                        exc_value.__traceback__ = traceback
                    # Only set the 'errors_occurred' flag for errors that may make
                    # the connection unusable.
                    if dj_exc_type not in (DataError, IntegrityError):
                        self.wrapper.errors_occurred = True
                    six.reraise(dj_exc_type, dj_exc_value, traceback)

        def __call__(self, func):
            # Note that we are intentionally not using @wraps here for performance
            # reasons. Refs #21109.
            def inner(*args, **kwargs):
                with self:
                    return func(*args, **kwargs)
            return inner

    def load_backend(backend_name):
        """
        Return a database backend's "base" module given a fully qualified database
        backend name, or raise an error if it doesn't exist.
        """
        # This backend was renamed in Django 1.9.
        if backend_name == 'django.db.backends.postgresql_psycopg2':
            backend_name = 'django.db.backends.postgresql'

        try:
            return import_module('%s.base' % backend_name)
        except ImportError as e_user:
            # The database backend wasn't found. Display a helpful error message
            # listing all possible (built-in) database backends.
            backend_dir = os.path.join(os.path.dirname(upath(__file__)), 'backends')
            try:
                builtin_backends = [
                    name for _, name, ispkg in pkgutil.iter_modules([npath(backend_dir)])
                    if ispkg and name not in {'base', 'dummy', 'postgresql_psycopg2'}
                ]
            except EnvironmentError:
                builtin_backends = []
            if backend_name not in ['django.db.backends.%s' % b for b in
                                    builtin_backends]:
                backend_reprs = map(repr, sorted(builtin_backends))
                error_msg = ("%r isn't an available database backend.\n"
                             "Try using 'django.db.backends.XXX', where XXX "
                             "is one of:\n    %s\nError was: %s" %
                             (backend_name, ", ".join(backend_reprs), e_user))
                raise ImproperlyConfigured(error_msg)
            else:
                # If there's some other error, this must be an error in Django
                raise

    class ConnectionDoesNotExist(Exception):
        pass

    class ConnectionHandler(object):
        def __init__(self, databases=None):
            """
            databases is an optional dictionary of database definitions (structured
            like settings.DATABASES).
            """
            self._databases = databases
            self._connections = local()

        @cached_property
        def databases(self):
            if self._databases is None:
                self._databases = settings.DATABASES
            if self._databases == {}:
                self._databases = {
                    DEFAULT_DB_ALIAS: {
                        'ENGINE': 'django.db.backends.dummy',
                    },
                }
            if self._databases[DEFAULT_DB_ALIAS] == {}:
                self._databases[DEFAULT_DB_ALIAS]['ENGINE'] = 'django.db.backends.dummy'

            if DEFAULT_DB_ALIAS not in self._databases:
                raise ImproperlyConfigured("You must define a '%s' database" % DEFAULT_DB_ALIAS)
            return self._databases

        def ensure_defaults(self, alias):
            """
            Puts the defaults into the settings dictionary for a given connection
            where no settings is provided.
            """
            try:
                conn = self.databases[alias]
            except KeyError:
                raise ConnectionDoesNotExist("The connection %s doesn't exist" % alias)

            conn.setdefault('ATOMIC_REQUESTS', False)
            conn.setdefault('AUTOCOMMIT', True)
            conn.setdefault('ENGINE', 'django.db.backends.dummy')
            if conn['ENGINE'] == 'django.db.backends.' or not conn['ENGINE']:
                conn['ENGINE'] = 'django.db.backends.dummy'
            conn.setdefault('CONN_MAX_AGE', 0)
            conn.setdefault('OPTIONS', {})
            conn.setdefault('TIME_ZONE', None)
            for setting in ['NAME', 'USER', 'PASSWORD', 'HOST', 'PORT']:
                conn.setdefault(setting, '')

        def prepare_test_settings(self, alias):
            """
            Makes sure the test settings are available in the 'TEST' sub-dictionary.
            """
            try:
                conn = self.databases[alias]
            except KeyError:
                raise ConnectionDoesNotExist("The connection %s doesn't exist" % alias)

            test_settings = conn.setdefault('TEST', {})
            for key in ['CHARSET', 'COLLATION', 'NAME', 'MIRROR']:
                test_settings.setdefault(key, None)

        def __getitem__(self, alias):
            if hasattr(self._connections, alias):
                return getattr(self._connections, alias)

            self.ensure_defaults(alias)
            self.prepare_test_settings(alias)
            db = self.databases[alias]
            backend = load_backend(db['ENGINE'])
            conn = backend.DatabaseWrapper(db, alias)
            setattr(self._connections, alias, conn)
            return conn

        def __setitem__(self, key, value):
            setattr(self._connections, key, value)

        def __delitem__(self, key):
            delattr(self._connections, key)

        def __iter__(self):
            return iter(self.databases)

        def all(self):
            return [self[alias] for alias in self]

        def close_all(self):
            for alias in self:
                try:
                    connection = getattr(self._connections, alias)
                except AttributeError:
                    continue
                connection.close()

    class ConnectionRouter(object):
        def __init__(self, routers=None):
            """
            If routers is not specified, will default to settings.DATABASE_ROUTERS.
            """
            self._routers = routers

        @cached_property
        def routers(self):
            if self._routers is None:
                self._routers = settings.DATABASE_ROUTERS
            routers = []
            for r in self._routers:
                if isinstance(r, six.string_types):
                    router = import_string(r)()
                else:
                    router = r
                routers.append(router)
            return routers

        def _router_func(action):
            def _route_db(self, model, **hints):
                chosen_db = None
                for router in self.routers:
                    try:
                        method = getattr(router, action)
                    except AttributeError:
                        # If the router doesn't have a method, skip to the next one.
                        pass
                    else:
                        chosen_db = method(model, **hints)
                        if chosen_db:
                            return chosen_db
                instance = hints.get('instance')
                if instance is not None and instance._state.db:
                    return instance._state.db
                return DEFAULT_DB_ALIAS
            return _route_db

        db_for_read = _router_func('db_for_read')
        db_for_write = _router_func('db_for_write')

        def allow_relation(self, obj1, obj2, **hints):
            for router in self.routers:
                try:
                    method = router.allow_relation
                except AttributeError:
                    # If the router doesn't have a method, skip to the next one.
                    pass
                else:
                    allow = method(obj1, obj2, **hints)
                    if allow is not None:
                        return allow
            return obj1._state.db == obj2._state.db

        def allow_migrate(self, db, app_label, **hints):
            for router in self.routers:
                try:
                    method = router.allow_migrate
                except AttributeError:
                    # If the router doesn't have a method, skip to the next one.
                    continue

                allow = method(db, app_label, **hints)

                if allow is not None:
                    return allow
            return True

        def allow_migrate_model(self, db, model):
            return self.allow_migrate(
                db,
                model._meta.app_label,
                model_name=model._meta.model_name,
                model=model,
            )

        def get_migratable_models(self, app_config, db, include_auto_created=False):
            """
            Return app models allowed to be synchronized on provided db.
            """
            models = app_config.get_models(include_auto_created=include_auto_created)
            return [model for model in models if self.allow_migrate_model(db, model)]