案例

# import fcntl
import sys


def singleton(cls):
    _instance = {}

    def inner():
        if cls not in _instance:
            _instance[cls] = cls()
        return _instance[cls]

    return inner


class Singleton(object):
    def __init__(self, cls):
        self._cls = cls
        self._instance = {}

    def __call__(self, *args, **kwargs):
        if self._cls not in self._instance:
            self._instance[self._cls] = self._cls(*args, **kwargs)
        return self._instance[self._cls]


class ClassSingle(type):
    _instances = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(ClassSingle, cls).__call__(*args, **kwargs)
        return cls._instances[cls]


# class ScriptSingle:
#     pid_file = 0
#     is_run = False
#
#     def __init__(self, filepath):
#         self.pid_file = open(filepath, "w")
#
#         try:
#             fcntl.flock(self.pid_file, fcntl.LOCK_EX | fcntl.LOCK_NB)
#         except BaseException:
#             self.is_run = True
#
#     def is_running(self):
#         return self.is_run


if __name__ == '__main__':
    @singleton
    class A:
        pass


    a, a1 = A(), A()
    print(id(a) == id(a1))


    @Singleton
    class B:
        pass


    b, b1 = B(), B()
    print(id(b) == id(b1))


    class C(metaclass=ClassSingle):
        pass


    c, c1 = C(), C()
    print(id(c) == id(c1))

    # s = ScriptSingle('single.pid')
    # if s.is_running():
    #     print('is running!')
    #     sys.exit(0)