Abstract Classes: ABC, @abstractmethod, Protocol

As codebases grow, you need mechanisms to define contracts — "any class that does X must implement Y". Python offers two approaches: ABC (Abstract Base Classes) for inheritance-based contracts, and Protocol for structural typing (duck typing with type safety).

ABC and abstractmethod

ABC from abc module is a base class that supports abstract methods. You can't instantiate a class with unimplemented abstract methods:

Concrete Subclasses Must Implement All Abstract Methods

A subclass becomes concrete (instantiatable) only when it provides implementations for all abstract methods:

@abstractproperty Pattern

Use @property combined with @abstractmethod to require properties in subclasses:

Protocol — Structural Subtyping

Protocol (from typing) defines an interface by structure — a class satisfies a Protocol if it has the right methods and attributes, regardless of its class hierarchy. No inheritance required:

@runtime_checkable Protocol

By default, Protocols only work with static type checkers. Add @runtime_checkable to enable isinstance() checks at runtime:

Note: @runtime_checkable only checks for the existence of methods, not their signatures.

ABC vs Protocol: When to Use Each