lazy_import.py

# Copyright 2015 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ==============================================================================

"""A LazyLoader class."""

import importlib
import types


class LazyLoader(types.ModuleType):
    """Lazily import a module, mainly to avoid pulling in large dependencies.

    `contrib`, and `ffmpeg` are examples of modules that are large and not always
    needed, and this allows them to only be loaded when they are used.

    Note: Subclassing types.ModuleType allow us to correctly adhere with sys.modules, import system
    """

    def __init__(self, local_name, parent_module_globals, name):  # pylint: disable=super-on-old-class
        self._local_name = local_name
        self._parent_module_globals = parent_module_globals

        super(LazyLoader, self).__init__(name)

    def _load(self):
        # Import the target module and insert it into the parent's namespace
        module = importlib.import_module(self.__name__)
        self._parent_module_globals[self._local_name] = module

        # Update this object's dict so that if someone keeps a reference to the
        #   LazyLoader, lookups are efficient (__getattr__ is only called on lookups
        #   that fail).
        self.__dict__.update(module.__dict__)

        return module

    def __getattr__(self, item):
        module = self._load()
        return getattr(module, item)

    def __dir__(self):
        module = self._load()
        return dir(module)