Return type incorrect for `XMLVerifier.verify` when `expect_references=1`

[samueljsb]

The return type for this method is always a union of a single result or a list of results:

# t.py
import signxml

data = object()
result = signxml.XMLVerifier().verify(data, expect_references=1)
reveal_type(result)

$ mypy t.py
t.py:6: note: Revealed type is "Union[signxml.verifier.VerifyResult, builtins.list[signxml.verifier.VerifyResult]]"
Success: no issues found in 1 source file

However, when we only expect one result, we only get one result, so this annotation is unhelpful. Callers shouldn't need to handle the list case.

I believe this can be fixed with typing.overload.

Would a PR be welcome to make that change?

[kislyuk]

Hi, sure, that sounds good. A PR would be welcome.

[samueljsb]

I've been working from a slightly out of date version and it looks like the expect_references argument has been deprecated and moved to a config object. Given that, I think the only way to make this return type annotation any better would be to make SignatureConfiguration generic on the value of expect_references, which sounds messy.

An up-to-date example of the reproduction:

# t.py
import signxml

data = object()
result = signxml.XMLVerifier().verify(
    data,
    expect_config=signxml.SignatureConfiguration(expect_references=1),
)
reveal_type(result)

$ mypy t.py
t.py:9: note: Revealed type is "Union[signxml.verifier.VerifyResult, builtins.list[signxml.verifier.VerifyResult]]"
Success: no issues found in 1 source file

[samueljsb]

I can't seem to make this work with generics and overloads, so this might not be solvable through mypy at the moment. 😕