How do abstract base classes (abc.ABC) enforce interface contracts in Python?
ABC and @abstractmethod block instantiation until every abstract method is implemented
abc.ABC is a class with ABCMeta as its metaclass. Methods decorated with @abstractmethod are recorded on the class, and ABCMeta refuses to instantiate any class that still has unimplemented abstract methods. That gives you a real interface contract: you get a TypeError at instantiation time, not a silent AttributeError later. ABCs also support virtual subclassing through register(), which lets a class be considered a subclass without inheritance. Note that abstractmethod does not prevent the method from having an implementation: subclasses can call super().method() to reuse shared behavior, and abstract properties and classmethods are also supported.
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Use @abstractmethod for methods subclasses must implement; ABCMeta enforces instantiation rules.
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Use @abstractproperty or property + abstractmethod for abstract attributes.
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Use register() or subclasshook for virtual subclasses when you cannot modify the target class.
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Trade-off: ABCs add runtime checks and a metaclass, which conflicts with other metaclasses. typing.Protocol offers structural typing as a lighter alternative.
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Common mistake: expecting @abstractmethod to prevent calling the method. It does not; it only prevents instantiation of incomplete subclasses.
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Common mistake: forgetting that an abstract class can still define concrete methods that subclasses inherit.
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Version note: abc.ABC has been available since 3.4. typing.Protocol with @runtime_checkable has been available since 3.8 for structural checks.
Scenario Questions
0-2 years experience
- 1You define an abstract class and try to instantiate it. What error appears and why?
- 2What does @abstractmethod do to a method?
2-5 years experience
- 1You want to reuse shared logic in an abstract method. How do subclasses call the parent implementation?
- 2You have a third-party class you cannot modify but want to treat it as a subclass of your ABC. How?
5-8 years experience
- 1You are designing a plugin interface. Would you use ABC, Protocol, or a plain base class with NotImplementedError?
- 2Your ABC uses a custom metaclass that conflicts with another base class. How do you resolve it?
8+ years experience
- 1Design an interface library that supports both nominal and structural typing, remains runtime-checkable, and does not impose metaclass conflicts.
- 2Explain how ABCMeta, __subclasshook__, and register() interact with isinstance and issubclass in large framework codebases.
Follow-up Questions
- What is the difference between abc.ABC and typing.Protocol?
- Why does instantiating a subclass with an unimplemented abstract method raise TypeError?