Typing interfaces

Question

What is the correct way to type an "interface" in python 3?

In the following sample:

class One(object):
    def foo(self) -> int:
        return 42


class Two(object):
    def foo(self) -> int:
        return 142


def factory(a: str):
    if a == "one":
        return One()

    return Two()

what would be the correct way to type the return value of the factory function?

It should be something like "A type with a single method named foo that accepts no arguments and returns an integer".

But not sure I can find how to do that.

UPD: this question is exclusively dedicated to typing.

Answer 1

You could use a typing.Union but, it sounds like you really want structural typing not nominal. Python supports this using typing.Protocol, which is a supported part of the python type-hinting system, so mypy will understand it, for example:

import typing

class Fooable(typing.Protocol):
    def foo(self) -> int:
        ...

class One(object):
    def foo(self) -> int:
        return 42


class Two(object):
    def foo(self) -> int:
        return 142


def factory(a: str) -> Fooable:
    if a == "one":
        return One()

    return Two()

x = factory('one')
x.foo()

Note, structural typing fits well with Python's duck-typing ethos. Python's typing system supports both structural and nominal forms.