ReviseAlgo Logo

Polymorphism & Abstraction

Abstract Classes

Defining interfaces with the abc module

Interview: Design patterns — tests understanding of interfaces, abstract methods, and enforcement of contracts

Last Updated: June 12, 2026 • 7 min read

Abstract classes define a common interface that subclasses must implement. Python provides the abc module (Abstract Base Classes) for this purpose. Abstract classes cannot be instantiated directly — they serve as blueprints for concrete subclasses. This enforces a contract that ensures all subclasses implement required methods.

Key Concepts

  • Inherit from ABC (or use ABCMeta metaclass)
  • Use @abstractmethod decorator to mark required methods
  • Cannot instantiate a class with unimplemented abstract methods
  • Abstract classes can have concrete methods (shared implementation)
  • Abstract properties with @property + @abstractmethod

When to Use Abstract Classes

  • Defining a common API for multiple implementations
  • Plugin systems where plugins must implement specific methods
  • Framework design: base classes that enforce implementation contracts
  • When duck typing isn't enough — you need compile-time/instantiation-time enforcement

Common Pitfall

Abstract methods with a default implementation still MUST be overridden in subclasses. The default body (even pass) is not automatically inherited — Python checks for explicit implementation.

Use Cases

Defining plugin interfaces that all plugins must implement

Repository pattern for data access layers (DB, API, in-memory)

Strategy pattern: interchangeable algorithms with common interface

Framework design: base handlers, serializers, validators

Enforcing contracts in team codebases (prevents forgetting to implement methods)

Common Mistakes

Forgetting to inherit from ABC — abstract methods won't be enforced

Not implementing all abstract methods — subclass can't be instantiated

Using abstract classes when a Protocol would be simpler (structural typing)

Putting too much logic in abstract base classes (violates interface segregation)

Forgetting that @property + @abstractmethod requires both decorators in correct order