11.1. Singleton¶
EN: Singleton
PL: Singleton
Type: object
11.1.1. Pattern¶
To ensure a class has a single instance
Database connection pool
HTTP Gateway
Settings
Main game/program window
11.1.2. Problem¶
from typing import Any
class ConfigManager:
settings: dict[str, Any]
def __init__(self) -> None:
self.settings = {}
def set(self, key: str, value: Any) -> None:
self.settings[key] = value
def get(self, key: str) -> Any:
return self.settings.pop(key)
if __name__ == '__main__':
manager = ConfigManager()
manager.set('name', 'Mark')
other = ConfigManager()
print(other.get('name'))
# Traceback (most recent call last):
# KeyError: 'name'
11.1.3. Solution¶
from __future__ import annotations
from typing import Any
class ConfigManager:
settings: dict[str, Any]
instance: ConfigManager | None = None
def __init__(self) -> None:
self.settings = {}
@classmethod
def get_instance(cls) -> ConfigManager:
if not cls.instance:
cls.instance = super().__new__(cls)
cls.instance.__init__()
return cls.instance
def set(self, key: str, value: Any) -> None:
self.settings[key] = value
def get(self, key: str) -> Any:
return self.settings.pop(key)
if __name__ == '__main__':
manager = ConfigManager.get_instance()
manager.set('name', 'Mark')
other = ConfigManager.get_instance()
print(other.get('name'))
# Mark
11.1.4. Use Case - 0x01¶
class Singleton:
instance = None
@classmethod
def get_instance(cls):
if not cls.instance:
cls.instance = ...
return cls.instance
# Creating first instance for the first time
first = Singleton.get_instance()
# Connecting for the second time
# Will use existing instance
second = Singleton.get_instance()
11.1.5. Use Case - 0x02¶
class Singleton:
instance = None
def __new__(cls, *args, **kwargs):
if not cls.instance:
cls.instance = super().__new__(*args, **kwargs)
obj = cls.instance
obj.__init__()
return obj
class MyClass(Singleton):
pass
11.1.6. Use Case - 0x03¶
class Database:
connection = None
@classmethod
def connect(cls):
if not cls.connection:
print('Establishing connection...')
cls.connection = ...
return cls.connection
# Connecting for the first time
# Will establish new connection
first = Database.connect()
# Connecting for the second time
# Will use existing connection to the DB
# The same handle as `first`
second = Database.connect()
11.1.7. Use Case - 0x04¶
class Singleton(type):
instances = {}
def __call__(cls, *args, **kwargs):
if cls not in cls.instances:
cls.instances[cls] = super().__call__(*args, **kwargs)
return cls.instances[cls]
class MyClass(metaclass=Singleton):
pass