What is a metaclass and when to use it?

Question

Accepted Answer

**Metaclass** is the **class of a class**. Controls how classes are created.

```python
class SingletonMeta(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 Database(metaclass=SingletonMeta):
    def __init__(self):
        self.connection = "connected"

db1 = Database()
db2 = Database()
print(db1 is db2)  # True — same instance

# Validation metaclass
class ValidatedMeta(type):
    def __new__(mcs, name, bases, namespace):
        for key, value in namespace.items():
            if isinstance(value, str) and not key.startswith("_"):
                if not key.isupper():
                    raise ValueError(f"{key} must be UPPERCASE")
        return super().__new__(mcs, name, bases, namespace)

class Config(metaclass=ValidatedMeta):
    API_KEY = "secret"  # OK
    # api_key = "secret"  # ValueError!
```

**When to use:**
- Singleton pattern.
- Class validation.
- Auto-registration.
- ORM frameworks.

**Prefer:** `__init_subclass__` for simpler cases.

Python: What is a metaclass and when to use it?