# Python Metaclasses

In Python, classes are objects. Just as an object is an instance of a class, a class is an instance of a **metaclass**. The default metaclass in Python is `type`.

## What is a Metaclass?

A metaclass is the class of a class. It controls the creation and behavior of classes.

*   **Object**: Instance of a Class.
*   **Class**: Instance of a Metaclass.

## The `type` Metaclass

`type` is the built-in metaclass that Python uses to create all classes by default.

```python
class MyClass:
    pass

obj = MyClass()

print(type(obj))      # Output: <class '__main__.MyClass'>
print(type(MyClass))  # Output: <class 'type'>
```

## Creating a Custom Metaclass

To create a custom metaclass, you must inherit from `type`. You typically override `__new__` or `__init__`.

*   `__new__(cls, name, bases, dct)`: Called to create the class.
*   `__init__(cls, name, bases, dct)`: Called to initialize the class.

```python
class MyMeta(type):
    def __new__(cls, name, bases, dct):
        print(f"Creating class: {name}")
        x = super().__new__(cls, name, bases, dct)
        x.custom_attribute = "Added by Metaclass"
        return x

class MyClass(metaclass=MyMeta):
    pass

# Output: Creating class: MyClass
print(MyClass.custom_attribute) # Output: Added by Metaclass
```

## Example: Singleton Pattern

You can use a metaclass to implement the Singleton pattern, ensuring a class only has one instance.

```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 Singleton(metaclass=SingletonMeta):
    pass
```

[[programming/python/python]]