mypy complains on "Missing return statement"

[Corwinpro]

Is this a known issue? What is the suggested way around this?

from result import Result, Err, Ok


def foo() -> Result[int, int]:
    return Ok(1)


def bar() -> str:
    match foo():
        case Ok(value):
            return f"{value}"
        case Err(e):
            raise RuntimeError(e)

Running mypy on this results in

example.py:8: error: Missing return statement  [return]

[wbolster]

this is called an ‘exhaustiveness check’ and it can be achieved with mypy but requires more code than your example. a web search yields various articles about the details.

also read up on typing.Never in the stdlib which has an example but a rather limited explanation: https://docs.python.org/3/library/typing.html#typing.Never

[Corwinpro]

@wbolster Interestingly, this passes?..

from result import Result, Err, Ok


def foo() -> Result[int, int]:
    return Ok(1)


def bar() -> str:
    result = foo()
    match result:
        case Ok(value):
            return f"{value}"
        case Err(e):
            raise RuntimeError(e)

[wbolster]

interesting, maybe mypy inference limitation? can you try reveal_type()?

[Corwinpro]

But this is not result-specific, I can see the same behaviour for any Union types, as in

def foo() -> int | str: return 1
def bar() -> int:
    match foo():  # <- does not work
        case int(i): return i
        case str(s): raise ValueError(s)
def baz() -> int:
    value = foo()
    match value:  # <- does work
        case int(i): return i
        case str(s): raise ValueError(s)

[francium]

This works,

def foo() -> int | str: return 1
def bar() -> int:
    match x := foo():
        case int(x): return x
        case str(x): raise ValueError(x)

[Corwinpro]

This is actually a mypy bug: python/mypy#12998