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