Generators not returning any result

[kellerza]

Hi, I have some Generators that might not yield a result.

How should this be typed?

from typeguard import typechecked
import typing

@typechecked
def empty() -> typing.Generator[int, None, None]:
    if False:
        yield 1

print(list(empty())
typeguard.TypeCheckError: the return value (None) is not an instance of collections.abc.Generator

[agronholm]

Given that empty() is a generator function, I'm not sure how this is happening. Perhaps the returned generator shim is the problem? Needs to be investigated.

[agronholm]

I can't reproduce this. Which version of Typeguard and Python are you using?

[kellerza]

Apologies, my example was over simplified. From the reproduction below, this happens when using an alias/TypeAlias
It might also be the actual return value of the Generator, instead of the absence of a yield.

Python 3.12.9
typeguard 4.4.2

Full example as a pytest test: uv run pytest -vv -k some

"""Test types."""

import pytest
from typing import Any, TypeAlias
from collections import abc
from typeguard import typechecked, TypeCheckError

RowDataGen = abc.Generator[dict[str, Any], None, None]


@typechecked
def return_some_bad(lst: list[str]) -> RowDataGen:
    """Return None."""
    for val in lst:
        yield ({"a": val})


@typechecked
def return_some_good(lst: list[str]) -> abc.Generator[dict[str, Any], None, None]:
    """Return None."""
    for val in lst:
        yield ({"a": val})


def test_return_some():
    """Test return_none."""
    assert list(return_some_good(["1", "2"])) == [{"a": "1"}, {"a": "2"}]
    assert list(return_some_good([])) == []

    with pytest.raises(TypeCheckError) as err:
        assert list(return_some_bad(["1", "2"])) == [{"a": "1"}, {"a": "2"}]
    assert "is not an instance of collections.abc.Generator" in str(err)

    with pytest.raises(TypeCheckError):
        assert list(return_some_bad([])) == []
    assert "is not an instance of collections.abc.Generator" in str(err)

[agronholm]

The test passes just fine over here.

[agronholm]

Ah, I think I misunderstood this. The isn't supposed to pass, right?

[agronholm]

Yeah...so the root problem here is the type alias. The AST transformer doesn't know that the return type is a generator type so it doesn't follow the proper code path for generator functions. There might be something I can do about this.