`abc`

The Python abc module provides infrastructure for defining abstract base classes (ABCs). It allows you to create classes that define a set of methods that must be created within any subclasses built from the abstract base class.

An ABC can also be used to register virtual subclasses, which can be checked using the isinstance() or issubclass() functions.

Here’s a quick example:

>>> from abc import ABC, abstractmethod

>>> class BaseABC(ABC):
...     @abstractmethod
...     def method(self):
...         pass
...

>>> class ConcreteClass(BaseABC):
...     def method(self):
...         print("Implementation of method")
...

>>> obj = ConcreteClass()
>>> obj.method()
Implementation of method

Key Features

Defining abstract base classes using ABC or ABCMeta
Declaring abstract methods and properties with the @abstractmethod decorator
Preventing instantiation until all abstract methods are implemented
Registering virtual subclasses with ABCMeta.register() without using inheritance
Customizing issubclass() checks via the .__subclasshook__() magic method
Supporting mixin-style ABCs in multiple inheritance hierarchies
Exposing ABC cache/version helpers via .get_cache_token()
Updating abstract method sets after class creation with .update_abstractmethods()

Frequently Used Classes and Functions

Object	Type	Description
`abc.ABC`	Class	Provides a helper base class for defining abstract base classes
`abc.abstractmethod()`	Decorator	Marks a method as abstract
`abc.ABCMeta`	Metaclass	Defines the metaclass for defining abstract base classes
`abc.ABCMeta.register()`	Method	Registers a class as a virtual subclass
`builtins.isinstance()`	Built-in Function	Checks whether an object is an instance of a class or of a tuple of classes
`builtins.issubclass()`	Built-in Function	Checks whether a class is a subclass of a class or of a tuple of classes

Examples

Define an abstract base class:

>>> from abc import ABC, abstractmethod

>>> class Vehicle(ABC):
...     @abstractmethod
...     def drive(self):
...         pass

>>> from abc import ABC, abstractmethod

>>> class Car:
...     def drive(self):
...         print("Car is driving")
...

>>> Vehicle.register(Car)
>>> issubclass(Car, Vehicle)
True
>>> isinstance(Car(), Vehicle)
True

Common Use Cases

The most common tasks for abc include:

Defining required APIs that concrete subclasses must implement
Preventing incomplete implementations from being instantiated
Building framework base classes and plugin interfaces with enforced override points
Validating objects and classes at runtime with isinstance() and issubclass()
Registering third-party classes as virtual subclasses when inheritance isn’t practical
Implementing structural interface checks via .__subclasshook__()
Creating mixins that provide shared behavior while still requiring concrete methods

Real-World Example

Suppose you’re building a plugin system where each plugin must implement a .run() method. You can use an ABC to enforce this requirement:

>>> from abc import ABC, abstractmethod

>>> class Plugin(ABC):
...     @abstractmethod
...     def run(self):
...         pass
...

>>> class CustomPlugin(Plugin):
...     def run(self):
...         print("Running custom plugin...")
...

>>> def execute_plugin(plugin):
...     plugin.run()
...

>>> plugin = CustomPlugin()
>>> execute_plugin(plugin)
Running custom plugin...

In this example, the abc module ensures that any class claiming to be a Plugin must implement the .run() method, providing a consistent interface for executing plugins.

abc