# Django 2.2.9.dev20191202104323 文档

# django.utils.module_loading 源代码

    import copy
    import os
    from importlib import import_module
    from importlib.util import find_spec as importlib_find

    def import_string(dotted_path):
        """
        Import a dotted module path and return the attribute/class designated by the
        last name in the path. Raise ImportError if the import failed.
        """
        try:
            module_path, class_name = dotted_path.rsplit('.', 1)
        except ValueError as err:
            raise ImportError("%s doesn't look like a module path" % dotted_path) from err

        module = import_module(module_path)

        try:
            return getattr(module, class_name)
        except AttributeError as err:
            raise ImportError('Module "%s" does not define a "%s" attribute/class' % (
                module_path, class_name)
            ) from err

    def autodiscover_modules(*args, **kwargs):
        """
        Auto-discover INSTALLED_APPS modules and fail silently when
        not present. This forces an import on them to register any admin bits they
        may want.

        You may provide a register_to keyword parameter as a way to access a
        registry. This register_to object must have a _registry instance variable
        to access it.
        """
        from django.apps import apps

        register_to = kwargs.get('register_to')
        for app_config in apps.get_app_configs():
            for module_to_search in args:
                # Attempt to import the app's module.
                try:
                    if register_to:
                        before_import_registry = copy.copy(register_to._registry)

                    import_module('%s.%s' % (app_config.name, module_to_search))
                except Exception:
                    # Reset the registry to the state before the last import
                    # as this import will have to reoccur on the next request and
                    # this could raise NotRegistered and AlreadyRegistered
                    # exceptions (see #8245).
                    if register_to:
                        register_to._registry = before_import_registry

                    # Decide whether to bubble up this error. If the app just
                    # doesn't have the module in question, we can ignore the error
                    # attempting to import it, otherwise we want it to bubble up.
                    if module_has_submodule(app_config.module, module_to_search):
                        raise

    def module_has_submodule(package, module_name):
        """See if 'module' is in 'package'."""
        try:
            package_name = package.__name__
            package_path = package.__path__
        except AttributeError:
            # package isn't a package.
            return False

        full_module_name = package_name + '.' + module_name
        try:
            return importlib_find(full_module_name, package_path) is not None
        except (ImportError, AttributeError):
            # When module_name is an invalid dotted path, Python raises ImportError
            # (or ModuleNotFoundError in Python 3.6+). AttributeError may be raised
            # if the penultimate part of the path is not a package.
            # (https://bugs.python.org/issue30436)
            return False

    def module_dir(module):
        """
        Find the name of the directory that contains a module, if possible.

        Raise ValueError otherwise, e.g. for namespace packages that are split
        over several directories.
        """
        # Convert to list because _NamespacePath does not support indexing on 3.3.
        paths = list(getattr(module, '__path__', []))
        if len(paths) == 1:
            return paths[0]
        else:
            filename = getattr(module, '__file__', None)
            if filename is not None:
                return os.path.dirname(filename)
        raise ValueError("Cannot determine directory containing %s" % module)