Python🚀Advancedadvanced

Metaclasses

Python · Advanced · Run with python3

Explanation

A metaclass is the class of a class — it controls how classes are created. `type` is the default metaclass of all Python classes. Custom metaclasses override `__new__` and `__init__` to intercept class creation: adding methods, validating class definitions, registering classes, or enforcing interfaces. The `__init_subclass__` hook (Python 3.6+) is a simpler alternative for many metaclass use cases. `abc.ABCMeta` is the most widely used metaclass — it powers `@abstractmethod`.

Real-World Scenario

Django's ORM uses a metaclass to transform class-level field declarations (`name = CharField(max_length=100)`) into a table schema and query interface. The metaclass intercepts class creation and replaces `CharField` instances with descriptors wired to the database. ```python class SingletonMeta(type): """Metaclass that ensures only one instance per class.""" _instances: dict = {} def __call__(cls, *args, **kwargs): if cls not in cls._instances: cls._instances[cls] = super().__call__(*args, **kwargs) return cls._instances[cls] class Config(metaclass=SingletonMeta): def __init__(self): self.debug = False self.version = "1.0" a = Config() b = Config() print(a is b) # True — same instance a.debug = True print(b.debug) # True — same object # __init_subclass__ — simpler registration pattern class Plugin: _registry: dict[str, type] = {} def __init_subclass__(cls, name: str = "", **kwargs): super().__init_subclass__(**kwargs) if name: Plugin._registry[name] = cls class AuthPlugin(Plugin, name="auth"): def run(self): return "auth" print(Plugin._registry) # {'auth': <class 'AuthPlugin'>} ```

Quick Snippet

snippet.py

python

class SingletonMeta(type):
    """Metaclass that ensures only one instance per class."""
    _instances: dict = {}

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

class Config(metaclass=SingletonMeta):
    def __init__(self):
        self.debug = False
        self.version = "1.0"

a = Config()
b = Config()
print(a is b)       # True — same instance
a.debug = True
print(b.debug)      # True — same object

# __init_subclass__ — simpler registration pattern
class Plugin:
    _registry: dict[str, type] = {}

    def __init_subclass__(cls, name: str = "", **kwargs):
        super().__init_subclass__(**kwargs)
        if name:
            Plugin._registry[name] = cls

class AuthPlugin(Plugin, name="auth"):
    def run(self): return "auth"

print(Plugin._registry)  # {'auth': <class 'AuthPlugin'>}

Full Example

View on GitHub

metaclasses.py

python

# Compile: python3 metaclasses.py
# Output:
# True
# True
# {'auth': <class '__main__.AuthPlugin'>}

class SingletonMeta(type):
    _instances: dict = {}

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

class Config(metaclass=SingletonMeta):
    def __init__(self):
        self.debug = False

a = Config()
b = Config()
print(a is b)       # True
a.debug = True
print(b.debug)      # True — same object

class Plugin:
    _registry: dict = {}

    def __init_subclass__(cls, name: str = "", **kwargs):
        super().__init_subclass__(**kwargs)
        if name:
            Plugin._registry[name] = cls

class AuthPlugin(Plugin, name="auth"):
    def run(self): return "auth"

print(Plugin._registry)

Run command

bash

python3 metaclasses.py

`asyncio.TaskGroup` and `ExceptionGroup` (Python 3.11+)

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Descriptors

Python