A type annotates a function parameter as derived from several abstract base classes in Python

Question

I am trying to introduce function annotation in Python 2 according to PEP 484 . The function accepts a container that must implement both __len__, and __iter__. The source code in which I want to add this annotation is quite complicated, so consider an example function that returns the product of all intin the container sif it len(s)is equal and returns 1 otherwise.

If I want to annotate a container where only it is required __len__, I would annotate it as type: (Sized) -> int. If I wanted to annotate a container where only needed __iter__, I would annotate it as type: (Iterable[int]) -> int. But how can I completely annotate a container in which I need both?

Edit:

I tried this as suggested by piotr-Ćwiek:

from __future__ import print_function
from typing import Sized, Iterable

class SizedIterable(Sized, Iterable[int]):
    pass

def product2(numbers):
    # type: (SizedIterable) -> int
    if len(numbers)%2 == 1:
        return 1
    else:
        p = 1
        for n in numbers:
            p*= n
        return p

print(product2([1, 2, 3, 4]))
print(product2({1, 2, 3, 4}))

but this failed with this error:

prod2.py:17: error: Argument 1 to "product2" has incompatible type List[int]; expected "SizedIterable"
prod2.py:18: error: Argument 1 to "product2" has incompatible type Set[int]; expected "SizedIterable"

Answer 1

In python 3.6 there exists typing.Collectionone that works almost perfectly for your use case (it also inferred from Container, but really everything you want to use is likely to have one __contains__). Unfortunately, there is no solution for python 2.

SizedIterable , , , Sized Iterable, mypy, ; mypy , , Sized Iterable SizedIterable.

Mypy ; , , :

class A(Sized, Iterable[int]):
  def g(self) -> None:
    ...

def f(x: A) -> None:
  a.g()

# passes type check because [1, 2] is Sized and Iterable
# but fails in run-time
f([1, 2])

, mypy , .

mypy , mypy .

:

• ( @PiotrĆwiek); , , Iterable Sized
• ; , , , , , numbers __len__ __iter__

typing __instancecheck__ , ( -) - mypy - , (.. , ..). , mypy - , , (, python, ..).