class

class inherit from object

class MyClass(object):
    # class code follows...

• Python 2, many reasons.

• Python 3, no reason.

The following 2 classes are complete the same.

class ClassicSpam:
    pass

class NewSpam(object):
    pass

ABC, Abstract Base Classes

http://www.thedigitalcatonline.com/blog/2016/04/03/abstract-base-classes-in-python/

[1] to resolve: no-type-checking in python

try:
    someobj[1]
except TypeError:
    # when object is not subscriptable

[2] define sync interface among parent and child class.

Define it

by real inheritance

class ChildClass(ParentClass):
    pass

by virtual subclass

ParentClass.register(ChildClass)

Registering a class does not imply any form of check about methods or attributes. Registering is just the promise that a given behaviour is provided by the registered class.

Create ABC

from abc import ABCMeta

class MyABC(metaclass=ABCMeta):
    pass

MyABC.register(tuple)

assert issubclass(tuple, MyABC)
assert isinstance((), MyABC)

Metaclass

# Metaclass definition
class NewType(type):
    def __init__(self, name, bases, namespace):
        self.answer = 42

# Link class and metaclass
class Child(metaclass=NewType): pass

# Use the class
assert Child.answer == 42

metaclass = mixin

standard case:

class GrandParent(): pass

class Parent(GrandParent): pass

class Child(Parent): pass

c = Child()

calling get_name() on c, and it will check Child -> Parent -> GrandParent -> object

metaclass case:

class NewType(type): pass

class GrandParent(): pass

class Parent(GrandParent): pass

class Child(Parent, metaclass=NewType): pass

c = Child()

calling get_name() on c, and it will check Child -> Parent -> GrandParent -> object -> NewType

>>> Child.mro()
[<class '__main__.Child'>, <class '__main__.Parent'>, <class '__main__.GrandParent'>, <class 'object'>]
>>> Child.__class__
<class '__main__.NewType'>
>>>

abstractmethod = class with interface

class MyABC(metaclass=abc.ABCMeta):
    @abc.abstractmethod
    def get(self):
        pass

>>> m = MyABC()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class MyABC with abstract methods get

class Concrete(MyABC):
    def get(self):
        return 1

>>> c = Concrete()
>>>

prevents the class from being instantiated if the method is not implemented