How is the list class a subclass of collections.abc.Sequence in Python?

Question

In Python, the list class seems to be a subclass of collections.abc.Sequence (which makes totally sense):

from collections.abc import Sequence
issubclass(list, Sequence)
# returns True

But the list type doesn't seem to inherit from Sequence:

dict.__mro__
# returns (dict, object)

So, how does this issubclass(list, Sequence) is working? How does it return True?

Answer 1

The collections.abc types overload the isinstance() and issubclass() operators to check if the object has the expected interface, without requiring true subclassing. See PEP 3119 which introduced abstract base classes; in particular the section on overloading those two functions for more. This is sort of the point of the collections.abc module. For example, if you define your own class that provides the expected attributes, Python will say that it is an instance/subclass of that type:

>>> class MyClass:
...   __contains__ = 0
...
>>> from collections import abc
>>> issubclass(MyClass, abc.Container)
True

The point of checking if something is a subclass or instance of some other type generally is to check that it has the expected interface. In this case, MyClass does have a __contains__ attribute, so it's assumed to conform to the interface. Of course in this example it doesn't actually conform to the interface: MyClass.__contains__ is not a method, so your program will fail if you try to use a MyClass() object as a container. However, issubclass() and isinstance() will tell you that it is a container object. The usual duck-typing rules apply.