Pylance doesn't resolve TypeVar type correctly

[smeznaric]

Environment data

• Language Server version: PyLance 2021.4.1
• OS and version: CentOS Linux release 7.5.1804
• Python version (& distribution if applicable, e.g. Anaconda): 3.6.3

Code Snippet / Additional information

I am using basic type checking (not strict).

from typing import TypeVar
import pandas as pd

DfOrSer = TypeVar("DfOrSer", pd.Series, pd.DataFrame)

def f(x: DfOrSer) -> DfOrSer:
    if isinstance(x, pd.DataFrame):
        return 2 * x
    else:
        assert isinstance(x, pd.Series)
        return 3 * x

Expected behaviour

I would expect no type issues with the above.

Actual behaviour

Both 2 * x and 3 * x are underlined.

2 * x says: Expression of type "DataFrame" cannot be assigned to return type "DfOrSer@f"
Type "DataFrame" cannot be assigned to type "DfOrSer@f" Pylance(reportGeneralTypeIssues)

3 * x says: Expression of type "Series[Unknown]" cannot be assigned to return type "DfOrSer@f"
Type "Series[Unknown]" cannot be assigned to type "DfOrSer@f" Pylance(reportGeneralTypeIssues)

I feel that generally the type checker catches too many false positives, which is reducing its utility in coding.

[erictraut]

Thanks for posting this sample. I'll look into it in more detail.

If you have other instances where you feel a false positive is generated — especially in "basic" type checking mode (as opposed to "strict" mode), please file bugs on those, and we'll look at them too.

[smeznaric]

Will do, thanks!

[erictraut]

Thanks again for the bug report.

This was a tricky bug in pyright, the type checker that powers pylance. It involved three of the most complex piece of code in the type checker: logic to handle constrained TypeVars, type narrowing for "isinstance" calls, and binary operations (in this case, a __rmul__ operation).

I've fixed the bug and beefed up the unit tests in this area. The fix will be in the next release of pylance.

[smeznaric]

Brilliant, thanks!

[erictraut]

We'll leave the bug open until the next release.

[jakebailey]

This issue has been fixed in version 2021.4.2, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202142-21-april-2021