Recognize functions like exit() as not returning

[JukkaL]

This generates a warning when using --warn-no-return, even though it's safe:

def f(x: int) -> int:
    if x == 0:
        return x
    exit(1)

It would be nice if mypy knew about at least the most common stdlib functions that never return such as sys.exit.

[elazarg]

Something like this?

NoReturn = Union[()]
def exit(n: int) -> NoReturn: ...

[JukkaL]

That is one way to do it, but it may be a little confusing.

[elazarg]

It's better than special casing exit(), which will not be robust and cannot be as flexible.

If naming is the problem, it can be solved (as above) from outside of mypy. Converging to naming conventions of another language such as [[noreturn]] in C++ and Nothing in Scala may benefit both languages.

[JukkaL]

Having a general mechanism would be more flexible, but that's harder since we'll need to standardize on a solution (this has been discussed before and needs a decision here: python/typing#165). Special casing a few functions like sys.exit would be a reasonable incremental improvement, at least for now.

[ddfisher]

Let's start by special casing the exit functions in the standard library. That might be good enough for a while.

[gvanrossum]

Here's a strategy that could work. Make mypy understand Union[()] as the magic, and update typeshed to mark up sys.exit(), os.exit() and builtins.exit() with `-> Union[()]`. We won't need changes to typing.py immediately (because this is only in the stubs) but we should still go ahead and update typing.py to allow Union[()] -- currently it doesn't allow this. We should also update PEP 484 to explain this. Since it's relatively uncommon apart from those few builtins maybe we won't need to pick a name for this -- Union[()] can be the idiomatic way to spell it.

[gvanrossum]

We're going to spell this as NoReturn, initially in mypy_extensions. See python/typing#165 (comment) and #2637.

[ilevkivskyi]

It looks like this issue is fixed now. At least the original example works correctly.

[ddfisher]

Yeah, this is fixed as of #2798.