Python singleton metaclass (thread-safe)

singleton.py

from __future__ import absolute_import, division, print_function, unicode_literals

from multiprocessing.dummy import Pool as ThreadPool
from multiprocessing import Lock
from threading import get_ident


class SingletonType(type):
    def __new__(mcs, name, bases, attrs):
        # Assume the target class is created (i.e. this method to be called) in the main thread.
        cls = super(SingletonType, mcs).__new__(mcs, name, bases, attrs)
        cls.__shared_instance_lock__ = Lock()
        return cls

    def __call__(cls, *args, **kwargs):
        with cls.__shared_instance_lock__:
            try:
                return cls.__shared_instance__
            except AttributeError:
                cls.__shared_instance__ = super(SingletonType, cls).__call__(*args, **kwargs)
                return cls.__shared_instance__


class NonThreadSafeSingletonType(type):
    def __call__(cls, *args, **kwargs):
        try:
            return cls.__shared_instance__
        except AttributeError:
            cls.__shared_instance__ = super(NonThreadSafeSingletonType, cls).__call__(*args, **kwargs)
            return cls.__shared_instance__


def run(DatabaseClass):
    result_set = set()
    result_set_access_lock = Lock()

    def worker(_result_set, _result_set_access_lock):
        db = DatabaseClass()
        with _result_set_access_lock:
            _result_set.add(db)

    thread_pool = ThreadPool(10)
    for i in six.moves.range(100):
        thread_pool.apply_async(worker, (result_set, result_set_access_lock))
    thread_pool.close()
    thread_pool.join()

    print(result_set)
    print(len(result_set))


if __name__ == "__main__":
    import six


    class DatabaseBase(object):
        def __init__(self):
            super(DatabaseBase, self).__init__()
            print("Created in thread: {}".format(get_ident()))

        def __repr__(self):
            return "<A database>"


    @six.add_metaclass(SingletonType)
    class Database(DatabaseBase):
        pass


    @six.add_metaclass(NonThreadSafeSingletonType)
    class NonThreadSafeDatabase(DatabaseBase):
        pass


    class NonSingletonDatabase(DatabaseBase):
        pass


    print("Thread-safe Singleton =============================")
    run(Database)
    print("Thread-safe Singleton =============================\n\n")

    print("Non-Thread-safe Singleton =========================")
    run(NonThreadSafeDatabase)
    print("Non-Thread-safe Singleton =========================\n\n")

    print("Normal ============================================")
    run(NonSingletonDatabase)
    print("Normal ============================================\n\n")

zzz-example-output

Thread-safe Singleton =============================
Created in thread: 123145307557888
{<A database>}
1
Thread-safe Singleton =============================


Non-Thread-safe Singleton =========================
Created in thread: 123145307557888
Created in thread: 123145312813056
Created in thread: 123145323323392
Created in thread: 123145339088896
Created in thread: 123145344344064
Created in thread: 123145354854400
Created in thread: 123145328578560
Created in thread: 123145349599232
Created in thread: 123145333833728
Created in thread: 123145318068224
{<A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>}
10
Non-Thread-safe Singleton =========================


Normal ============================================
Created in thread: 123145307557888
Created in thread: 123145312813056
Created in thread: 123145323323392
Created in thread: 123145333833728
Created in thread: 123145344344064
Created in thread: 123145354854400
Created in thread: 123145318068224
Created in thread: 123145339088896
Created in thread: 123145307557888
Created in thread: 123145349599232
Created in thread: 123145312813056
Created in thread: 123145328578560
Created in thread: 123145323323392
Created in thread: 123145333833728
Created in thread: 123145344344064
Created in thread: 123145354854400
Created in thread: 123145318068224
Created in thread: 123145339088896
Created in thread: 123145307557888
Created in thread: 123145349599232
Created in thread: 123145312813056
Created in thread: 123145328578560
Created in thread: 123145323323392
Created in thread: 123145333833728
Created in thread: 123145344344064
Created in thread: 123145354854400
Created in thread: 123145318068224
Created in thread: 123145339088896
Created in thread: 123145307557888
Created in thread: 123145349599232
Created in thread: 123145312813056
Created in thread: 123145328578560
Created in thread: 123145323323392
Created in thread: 123145333833728
Created in thread: 123145344344064
Created in thread: 123145354854400
Created in thread: 123145318068224
Created in thread: 123145339088896
Created in thread: 123145307557888
Created in thread: 123145349599232
Created in thread: 123145312813056
Created in thread: 123145328578560
Created in thread: 123145323323392
Created in thread: 123145333833728
Created in thread: 123145344344064
Created in thread: 123145354854400
Created in thread: 123145318068224
Created in thread: 123145339088896
Created in thread: 123145307557888
Created in thread: 123145349599232
Created in thread: 123145312813056
Created in thread: 123145328578560
Created in thread: 123145333833728
Created in thread: 123145344344064
Created in thread: 123145354854400
Created in thread: 123145323323392
Created in thread: 123145307557888
Created in thread: 123145349599232
Created in thread: 123145339088896
Created in thread: 123145333833728
Created in thread: 123145344344064
Created in thread: 123145312813056
Created in thread: 123145328578560
Created in thread: 123145349599232
Created in thread: 123145339088896
Created in thread: 123145354854400
Created in thread: 123145323323392
Created in thread: 123145307557888
Created in thread: 123145328578560
Created in thread: 123145333833728
Created in thread: 123145344344064
Created in thread: 123145312813056
Created in thread: 123145318068224
Created in thread: 123145349599232
Created in thread: 123145339088896
Created in thread: 123145333833728
Created in thread: 123145344344064
Created in thread: 123145328578560
Created in thread: 123145354854400
Created in thread: 123145323323392
Created in thread: 123145339088896
Created in thread: 123145333833728
Created in thread: 123145312813056
Created in thread: 123145318068224
Created in thread: 123145354854400
Created in thread: 123145307557888
Created in thread: 123145344344064
Created in thread: 123145328578560
Created in thread: 123145312813056
Created in thread: 123145323323392
Created in thread: 123145339088896
Created in thread: 123145333833728
Created in thread: 123145344344064
Created in thread: 123145318068224
Created in thread: 123145354854400
Created in thread: 123145307557888
Created in thread: 123145328578560
Created in thread: 123145323323392
Created in thread: 123145312813056
Created in thread: 123145349599232
{<A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>, <A database>}
100
Normal ============================================