Metaclass

Learn metaclasses with class Meta(type).

What is a metaclass?

A metaclass is a class that produces classes. You write your own with class Meta(type). When a class is defined with metaclass=Meta, its creation is controlled by Meta.__new__.

The metaclass overrides __new__(mcs, name, bases, ns) where ns is the class namespace. You can add attributes, change methods. type itself is a metaclass; all classes derive from it.

Changing class structure with new

In a metaclass __new__ runs before the class object is created. You can add entries to the namespace dict (ns); they become class attributes.

For example ns["created_at"] = "meta" makes created_at appear on the class. You call super().__new__(mcs, name, bases, ns) to produce the actual class object. This pattern powers tools like the Django ORM.

Exercise Build a class with the Meta metaclass

Write a Meta(type) metaclass. In __new__(mcs, name, bases, ns) add the key "created_at" with value "meta" to ns, then return super().__new__. Define a User class with metaclass=Meta. Print User.created_at.

Show example solution

# Write the Meta metaclass
class Meta(type):
    def __new__(mcs, name, bases, ns):
        ns["created_at"] = "meta"
        return super().__new__(mcs, name, bases, ns)

class User(metaclass=Meta):
    pass

print(User.created_at)

Test yourself

Complete the metaclass definition

class Meta(____):
    pass

Show answer

class Meta(type):
    pass

When is a metaclass __new__ called?
```
class Meta(type):
    def __new__(mcs, *a):
        print("creating")
        return super().__new__(mcs, *a)

class A(metaclass=Meta):
    pass
```
- When class A is defined
- When A() is called
- When the program starts
- When Meta() is called
Show answer

When Meta() is called — __new__ runs during class definition, i.e. when Meta is called.
Match metaclass concepts

Meta

Derives from type
__new__

Produces the class
mcs

The metaclass itself
ns

Namespace dict

Next lesson: __init_subclass__

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