Mypy: typing for infinite iterator?

[asottile]

Along the same lines as NoReturn, but for iterators. Here's the simplest case I could come up with:

import itertools

def f() -> int:
    for x in itertools.repeat(4):
        return 4

it's a toy example, but always returns, there's no way out of the loop

mypy currently flags this as:

$ mypy --version
mypy 0.641
$ mypy t.py 
t.py:4: error: Missing return statement

the workaround is to do something like this:

 def f() -> int:
     for x in itertools.repeat(4):
         return 4
+    raise AssertionError('unreachable!')

I don't really have any suggestions for how to represent such a thing, or if it would be actually useful in practice -- I do note that mypy already has ~some concept of infinite loops that could potentially be reapplied here:

# no error
def f() -> int:
    while True:
        for x in itertools.repeat(4):
            return x

[gvanrossum]

Honestly, such code seems really rare, and apart from toy examples like this, it's also hard to analyze for a human. The convention for an unreachable point in the code is assert False (which it seems mypy understands).

[asottile]

or if it would be actually useful in practice

yeah that was my thought as well -- hit this during advent of code fwiw :)

[dimaqq]

I've just written this "rare" code 😓

for delay in (10 ** i for in itertools.count(-3, 0.5)):
    if give_up:
        raise Timeout()
    try:
        return await something(...)
    except Unlucky:
        pass

    await asyncio.sleep(delay)

[yxliang01]

@dimaqq I also think this is not rare haha. Maybe you want to upvote #7374 or make a PR :)