# documentación de Django - 1.10.7

# Código fuente para django.utils.module_loading

    import copy
    import os
    import sys
    from importlib import import_module

    from django.utils import six

    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:
            msg = "%s doesn't look like a module path" % dotted_path
            six.reraise(ImportError, ImportError(msg), sys.exc_info()[2])

        module = import_module(module_path)

        try:
            return getattr(module, class_name)
        except AttributeError:
            msg = 'Module "%s" does not define a "%s" attribute/class' % (
                module_path, class_name)
            six.reraise(ImportError, ImportError(msg), sys.exc_info()[2])

    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

    if six.PY3:
        from importlib.util import find_spec as importlib_find

        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
            return importlib_find(full_module_name, package_path) is not None

    else:
        import imp

        def module_has_submodule(package, module_name):
            """See if 'module' is in 'package'."""
            name = ".".join([package.__name__, module_name])
            try:
                # None indicates a cached miss; see mark_miss() in Python/import.c.
                return sys.modules[name] is not None
            except KeyError:
                pass
            try:
                package_path = package.__path__   # No __path__, then not a package.
            except AttributeError:
                # Since the remainder of this function assumes that we're dealing with
                # a package (module with a __path__), so if it's not, then bail here.
                return False
            for finder in sys.meta_path:
                if finder.find_module(name, package_path):
                    return True
            for entry in package_path:
                try:
                    # Try the cached finder.
                    finder = sys.path_importer_cache[entry]
                    if finder is None:
                        # Implicit import machinery should be used.
                        try:
                            file_, _, _ = imp.find_module(module_name, [entry])
                            if file_:
                                file_.close()
                            return True
                        except ImportError:
                            continue
                    # Else see if the finder knows of a loader.
                    elif finder.find_module(name):
                        return True
                    else:
                        continue
                except KeyError:
                    # No cached finder, so try and make one.
                    for hook in sys.path_hooks:
                        try:
                            finder = hook(entry)
                            # XXX Could cache in sys.path_importer_cache
                            if finder.find_module(name):
                                return True
                            else:
                                # Once a finder is found, stop the search.
                                break
                        except ImportError:
                            # Continue the search for a finder.
                            continue
                    else:
                        # No finder found.
                        # Try the implicit import machinery if searching a directory.
                        if os.path.isdir(entry):
                            try:
                                file_, _, _ = imp.find_module(module_name, [entry])
                                if file_:
                                    file_.close()
                                return True
                            except ImportError:
                                pass
                        # XXX Could insert None or NullImporter
            else:
                # Exhausted the search, so the module cannot be found.
                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)