How do abstract base classes (ABCs) use a metaclass to enforce that subclasses implement required methods?

The abc module's ABCMeta metaclass tracks methods decorated with @abstractmethod, and when you attempt to instantiate a class (not subclass, but instantiate) that hasn't overridden all abstract methods, ABCMeta's __call__ raises TypeError before __init__ even runs. This enforces an interface contract at instantiation time rather than only failing later when a missing method is actually called.

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self): ...

class Circle(Shape):
    def __init__(self, r): self.r = r
    def area(self): return 3.14159 * self.r ** 2

try:
    Shape()  # TypeError: Can't instantiate abstract class
except TypeError as e:
    print(e)