Skip to content

Multiton Pattern

The Multiton pattern is a design pattern that generalizes the Singleton pattern by allowing multiple instances of a class, each identified by a unique key. This pattern is useful when you need to manage a limited number of instances that are distinguished by some attribute.

Import packages

1
2
3
import sqlite3
import threading
from typing import Any, Dict, Type

Define class

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
class Multiton(type):
    _instances: Dict[int, Any] = {}
    _locks: Dict[int, threading.Lock] = {}
    _global_lock: threading.Lock = threading.Lock()

    def __call__(cls, *args: Any, **kwargs: Any) -> Any:
        key = hash((cls, args, frozenset(kwargs.items())))

        if key in cls._instances:
            return cls._instances[key]

        with cls._global_lock:
            lock = cls._locks.setdefault(key, threading.Lock())

        with lock:
            if key not in cls._instances:
                cls._instances[key] = super().__call__(*args, **kwargs)

        return cls._instances[key]

    @staticmethod
    def drop() -> None:
        Multiton._instances.clear()
        Multiton._locks.clear()

Usage

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
class APIClient(metaclass=Multiton):
    def __init__(self, base_url: str):
        self.base_url = base_url


client1 = APIClient("https://service1.com")
client2 = APIClient("https://service1.com")
client3 = APIClient("https://service2.com")

print(client1 is client2)
print(client1 is client3)

True

False

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
class Config(metaclass=Multiton):
    def __init__(self, env: str):
        self.env = env


dev_config1 = Config("dev")
dev_config2 = Config("dev")
prod_config = Config("prod")

print(dev_config1 is dev_config2)
print(dev_config1 is prod_config)

True

False