# singleton.py

import pymongo

from functools import wraps

def singleton(cls):
    _instance = {}

    @wraps(cls)
    def get_instance(*args, **kwargs):
        if cls not in _instance:
            _instance[cls] = cls( *args, **kwargs)
        return _instance[cls]

    return get_instance

@singleton
class MyMongoClient(pymongo.MongoClient):
    def __str__(self):
        return (‘this is a 单例模式的应用‘)


if __name__ == ‘__main__‘:
        cli = MyMongoClient()
        print(cli)
        cli2 = MyMongoClient()
        print( cli is cli2)

(py3) jfxu@iZbp1gmf2s484lvojwrs4dZ:~$ python singleton.py
this is a 单例模式的应用
True

# singleton.py

import pymongo

class MyMongoClient(pymongo.MongoClient):

    _instance = None
    def __new__(cls,  *args, **kwargs):
        if not cls._instance:
            cls._instance = super(MyMongoClient, cls).__new__(cls, *args, **kwargs)
        return cls._instance

    def __str__(self):
        return ‘this is a 单例模式‘


if __name__ == ‘__main__‘:
    cli = MyMongoClient()
    print(cli)
    cli2 = MyMongoClient()
    print(cli is cli2)

(py3) jfxu@iZbp1gmf2s484lvojwrs4dZ:~$ python a.py
this is a 单例模式
True