1#!/usr/bin/env python3 2# Copyright 2019 - The Android Open Source Project 3# 4# Licensed under the Apache License, Version 2.0 (the "License"); 5# you may not use this file except in compliance with the License. 6# You may obtain a copy of the License at 7# 8# http://www.apache.org/licenses/LICENSE-2.0 9# 10# Unless required by applicable law or agreed to in writing, software 11# distributed under the License is distributed on an "AS IS" BASIS, 12# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13# See the License for the specific language governing permissions and 14# limitations under the License. 15 16"""A meta class for singleton pattern.""" 17 18 19class Singleton(type): 20 """A singleton metaclass. 21 22 Class attributes: 23 _instances: A dictionary keeps singletons' information, whose key is 24 class and value is an instance of that class. 25 26 Usage: 27 from aidegen.lib import singleton 28 29 class AClass(BaseClass, metaclass=singleton.Singleton): 30 pass 31 """ 32 33 _instances = {} 34 35 def __call__(cls, *args, **kwds): 36 """Initialize a singleton instance.""" 37 if cls not in cls._instances: 38 cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwds) 39 return cls._instances[cls] 40