# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το django.apps.config

    import os
    from importlib import import_module

    from django.core.exceptions import AppRegistryNotReady, ImproperlyConfigured
    from django.utils._os import upath
    from django.utils.module_loading import module_has_submodule

    MODELS_MODULE_NAME = 'models'

    class AppConfig(object):
        """
        Class representing a Django application and its configuration.
        """

        def __init__(self, app_name, app_module):
            # Full Python path to the application eg. 'django.contrib.admin'.
            self.name = app_name

            # Root module for the application eg. <module 'django.contrib.admin'
            # from 'django/contrib/admin/__init__.pyc'>.
            self.module = app_module

            # The following attributes could be defined at the class level in a
            # subclass, hence the test-and-set pattern.

            # Last component of the Python path to the application eg. 'admin'.
            # This value must be unique across a Django project.
            if not hasattr(self, 'label'):
                self.label = app_name.rpartition(".")[2]

            # Human-readable name for the application eg. "Admin".
            if not hasattr(self, 'verbose_name'):
                self.verbose_name = self.label.title()

            # Filesystem path to the application directory eg.
            # u'/usr/lib/python2.7/dist-packages/django/contrib/admin'. Unicode on
            # Python 2 and a str on Python 3.
            if not hasattr(self, 'path'):
                self.path = self._path_from_module(app_module)

            # Module containing models eg. <module 'django.contrib.admin.models'
            # from 'django/contrib/admin/models.pyc'>. Set by import_models().
            # None if the application doesn't have a models module.
            self.models_module = None

            # Mapping of lower case model names to model classes. Initially set to
            # None to prevent accidental access before import_models() runs.
            self.models = None

        def __repr__(self):
            return '<%s: %s>' % (self.__class__.__name__, self.label)

        def _path_from_module(self, module):
            """Attempt to determine app's filesystem path from its module."""
            # See #21874 for extended discussion of the behavior of this method in
            # various cases.
            # Convert paths to list because Python 3's _NamespacePath does not
            # support indexing.
            paths = list(getattr(module, '__path__', []))
            if len(paths) != 1:
                filename = getattr(module, '__file__', None)
                if filename is not None:
                    paths = [os.path.dirname(filename)]
                else:
                    # For unknown reasons, sometimes the list returned by __path__
                    # contains duplicates that must be removed (#25246).
                    paths = list(set(paths))
            if len(paths) > 1:
                raise ImproperlyConfigured(
                    "The app module %r has multiple filesystem locations (%r); "
                    "you must configure this app with an AppConfig subclass "
                    "with a 'path' class attribute." % (module, paths))
            elif not paths:
                raise ImproperlyConfigured(
                    "The app module %r has no filesystem location, "
                    "you must configure this app with an AppConfig subclass "
                    "with a 'path' class attribute." % (module,))
            return upath(paths[0])

        @classmethod
        def create(cls, entry):
            """
            Factory that creates an app config from an entry in INSTALLED_APPS.
            """
            try:
                # If import_module succeeds, entry is a path to an app module,
                # which may specify an app config class with default_app_config.
                # Otherwise, entry is a path to an app config class or an error.
                module = import_module(entry)

            except ImportError:
                # Track that importing as an app module failed. If importing as an
                # app config class fails too, we'll trigger the ImportError again.
                module = None

                mod_path, _, cls_name = entry.rpartition('.')

                # Raise the original exception when entry cannot be a path to an
                # app config class.
                if not mod_path:
                    raise

            else:
                try:
                    # If this works, the app module specifies an app config class.
                    entry = module.default_app_config
                except AttributeError:
                    # Otherwise, it simply uses the default app config class.
                    return cls(entry, module)
                else:
                    mod_path, _, cls_name = entry.rpartition('.')

            # If we're reaching this point, we must attempt to load the app config
            # class located at <mod_path>.<cls_name>
            mod = import_module(mod_path)
            try:
                cls = getattr(mod, cls_name)
            except AttributeError:
                if module is None:
                    # If importing as an app module failed, that error probably
                    # contains the most informative traceback. Trigger it again.
                    import_module(entry)
                else:
                    raise

            # Check for obvious errors. (This check prevents duck typing, but
            # it could be removed if it became a problem in practice.)
            if not issubclass(cls, AppConfig):
                raise ImproperlyConfigured(
                    "'%s' isn't a subclass of AppConfig." % entry)

            # Obtain app name here rather than in AppClass.__init__ to keep
            # all error checking for entries in INSTALLED_APPS in one place.
            try:
                app_name = cls.name
            except AttributeError:
                raise ImproperlyConfigured(
                    "'%s' must supply a name attribute." % entry)

            # Ensure app_name points to a valid module.
            app_module = import_module(app_name)

            # Entry is a path to an app config class.
            return cls(app_name, app_module)

        def check_models_ready(self):
            """
            Raises an exception if models haven't been imported yet.
            """
            if self.models is None:
                raise AppRegistryNotReady(
                    "Models for app '%s' haven't been imported yet." % self.label)

        def get_model(self, model_name):
            """
            Returns the model with the given case-insensitive model_name.

            Raises LookupError if no model exists with this name.
            """
            self.check_models_ready()
            try:
                return self.models[model_name.lower()]
            except KeyError:
                raise LookupError(
                    "App '%s' doesn't have a '%s' model." % (self.label, model_name))

        def get_models(self, include_auto_created=False, include_swapped=False):
            """
            Returns an iterable of models.

            By default, the following models aren't included:

            - auto-created models for many-to-many relations without
              an explicit intermediate table,
            - models created to satisfy deferred attribute queries,
            - models that have been swapped out.

            Set the corresponding keyword argument to True to include such models.
            Keyword arguments aren't documented; they're a private API.
            """
            self.check_models_ready()
            for model in self.models.values():
                if model._meta.auto_created and not include_auto_created:
                    continue
                if model._meta.swapped and not include_swapped:
                    continue
                yield model

        def import_models(self, all_models):
            # Dictionary of models for this app, primarily maintained in the
            # 'all_models' attribute of the Apps this AppConfig is attached to.
            # Injected as a parameter because it gets populated when models are
            # imported, which might happen before populate() imports models.
            self.models = all_models

            if module_has_submodule(self.module, MODELS_MODULE_NAME):
                models_module_name = '%s.%s' % (self.name, MODELS_MODULE_NAME)
                self.models_module = import_module(models_module_name)

        def ready(self):
            """
            Override this method in subclasses to run code when Django starts.
            """