Introduction to Python Metaclasses

Metaclasses are one of those Python features that sound complicated but are actually based on a simple idea:

Classes create objects.
Metaclasses create classes.

That’s it.

When you write:

class User:
    pass

the class User is actually an object, and Python needs something to build that object.
By default, Python uses the metaclass called type:

print(type(User))

Output:

<class 'type'>

So type is the “class-maker.”

Why Would You Ever Use a Metaclass?

Most people never do — and that’s fine.

But frameworks like Django, SQLAlchemy, and Pydantic use metaclasses to:

add extra attributes to classes

validate or modify classes at creation time

automatically register subclasses

build features with less boilerplate

A metaclass lets you run logic when the class is created, not when it runs.

A Basic Metaclass Example

This metaclass prints the name of the class being created:

class LoggerMeta(type):
    def __new__(cls, name, bases, attrs):
        print("Creating:", name)
        return super().__new__(cls, name, bases, attrs)

Using it:

class Example(metaclass=LoggerMeta):
    pass

When the file loads, it prints:

Creating: Example

Because the metaclass runs before the class exists.

Adding Automatic Attributes

A more practical example:

class InfoMeta(type):
    def __new__(cls, name, bases, attrs):
        attrs["source"] = "auto"
        return super().__new__(cls, name, bases, attrs)

Usage:

class Product(metaclass=InfoMeta):
    pass

print(Product.source)

Output:

auto

The metaclass quietly added the attribute.

When to Use Metaclasses

Use them only when you need to control how classes are built — usually in libraries or frameworks.

Normal applications almost never need them.