kwargs in the Base Class Tuple in Python

Question

I have noticed the snippet posed here, which is:

class Iterable(Generic[T], extra=collections_abc.Iterable): # (*)
    pass

class Iterator(Iterable, extra=collections_abc.Iterator):  
    pass

Question 1: How does extra=... work in this snippet? Or more generally, when can we put a keyword argument when subclassing? What does it do?

Question 2: As is mentioned in the discussion in the hub, the first class (marked with *) is a generic class parametrized on the type variable T. However, I am looking for something like this:

from typing import TypeVar, Generic

T = TypeVar('T')

class X(Generic[T]):
    a = T

class Y(X[int]):
    pass

Z = X[int]

print(Y.p) # gives "~T", but what I would like is "int"
print(Z.p) # also gives "~T"

In words, how to get a concrete subclass where the parameter T is fixed instead of the generic subclass with T undetermined? My first thoughts are something like these:

T = TypeVar(T)
class X(Generic[T]):
    p = T

class F(X[T], arg, **kwargs):  # as a functor with kwargs being the params? 

    # Do something to substitute T with arg
    '''
        This I suppose is incorrect and it is majorly 
    for this that I asked the first question.
    '''
    pass

# so that I can do
class Y(F, arg=P):
    pass

I looked into type(name, bases, dict) reference about dynamical class creation (even though the situation can be static) but I failed to find any specification on the bases argument about putting a tuple (kwargs) in the args.

kwargs in the Base Class Tuple in Python

0 Answers