abc module in Python

• Introduction
• abc Module
• collections.abc Module
• Animal example
• AwesomeSequence
• Recommended reading:

Introduction

abc is short for Abstract Base Class. Usually we use it to create a base class and force the subclass to implement certain functions. In python, abc is similar to C++’s abstract class.

There are three ways we can create this kind of class.

from abc import ABC, ABCMeta
from abc import abstractmethod

class Base1(ABC):
    @abstractmethod
    def hi(self):
         raise NotImplementedError("HH")

class Base2(metaclass=ABCMeta):
    @abstractmethod
    def hi(self):
        raise NotImplementedError("HH")

class Base3(object):
    __metaclass__ = ABCMeta
    @abstractmethod
    def hi(self):
        raise NotImplementedError("HH")

abc Module

• ABCMeta
  • This should be the meta class of any class that wants to be the abc.
• ABC
  • Just a simple wrapper, its meta class = ABCMeta. Users can directly inherit from it.
• ABCMeta.register
  • Dynamically register any class and issubclass, isinstance will return True after registration.
  • Will not affect the MRO mechanism
• abstractmethod:
  • If working with other decorators, please make sure that this is the first one. For example working with property or classmethod

collections.abc Module

Reference

Class	Inherits from	Required methods	Mixin methods
Sized		__len__
Awaitable		__await__
Coroutine	Awaitable	send;throw	close
AsyncIterable		__aiter__
AsyncIterator	AsyncIterable	__anext__	__aiter__
ASyncGenerator	AsyncIterator	asend, athrow	aclose
Iterable		__iter__
Callable		__call__
Hashable		__hash__
Reversable	Iterable	__reversed__
Iterator	Iterable	__next__	__iter__
Generator	Iterator	send, throw	close, __iter__, __next__
Container		__contains__
MutableSequence
Collection	Container Iterable Sized	__contains__ __iter__ __len__
Sequence	Reversable Collection	__getitem__;__len__	__contains__;__iter__ __reversed__;index;count
MutableSequence	Sequence	__getitem__;__setitem__ __delitem__;__len__;insert	Inherited from Sequence,append reverse,extend, pop, remove, __iadd__
ByteString	Sequence	__getitem__, __len__	Inherit from Sequence
Set	Collection	__contains__ __iter__,__len__	__le__, __lt__, __eq__, __ne__, __gt__, __ge__ __and__, __or__, __xor__, __sub__, isdisjoint
MutableSet	Set	__contains__ __iter__, __len__, add, discard	Inhert from Set, clear, pop remove __ior__, __iand__, __ixor__, __isub__
Mapping	Collection	__getitem__, __iter__, __len__	__contains__, keys, items, values get, __eq__, __ne__
MutableMapping	Mapping	__getitem__, __setitem__ __delitem__ __iter__, __len__	pop, popitem clear, update, setdefault

*View related base classes are ommited.

Animal example

from abc import ABC, abstractmethod

class Animal(ABC):

    @abstractmethod
    def talk(self):
        pass

class Dog(Animal):

    def talk(self):
        print("Wang!")

class Cat(Animal):

    def talk(self):
        print("Meow!")


class Snake(object):

    def x(self):
        print("x")

Animal.register(Snake)

class Bird(Animal):
    pass

Dog().talk()
Cat().talk()
print(issubclass(Snake, Animal)) # True
print(isinstance(Snake(), Animal)) # True
# TypeError: Can't instantiate abstract class Bird with abstract methods talk
bird = Bird()

AwesomeSequence

from collections.abc import Iterable

class AwesomeSequence(Iterable, Callable):

    def __init__(self):
        self._a = 1
        self._b = 0
        self._c = 2
        self._d = 4

    def __iter__(self):
        yield self._a
        yield self._b
        yield self._c
        yield self._d

    def __call__(self):
        return iter(self)

c = AwesomeSequence()
for v in iter(c):
    print(v)

for v in c():
    print(v)

Recommended reading:

• Python magic methods