Type subscript runtime exception false positive

[sbdchd]

Environment data

• Language Server version: 2021.4.0
• OS and version: darwin x64
• Python version (and distribution if applicable, e.g. Anaconda):
• python.analysis.indexing: undefined
• python.analysis.typeCheckingMode: off

Expected behaviour

Shouldn't report that indexing type will generate a runtime exception since the type isn't evaluated at runtime

Actual behaviour

error:

Subscript for class "OrderedDict" will generate runtime exception; enclose type annotation in quotes

Logs

Python Language Server Log

[Info  - 6:34:52 PM] Pylance language server 2021.4.0 (pyright 2918da1d) starting
[Info  - 6:34:52 PM] Server root directory: ~/.vscode/extensions/ms-python.vscode-pylance-2021.4.0/dist

Code Snippet / Additional information

if we run the following snippet it doesn't generate a runtime exception

from collections import OrderedDict


def main() -> None:
    x: OrderedDict[str, str] = OrderedDict()
    print(x)


main()

[erictraut]

It's very strange that it does not generate a runtime exception in the context of a variable type annotation, but it does when used in the context of a return type or parameter type annotation.

If I modify your code as follows and then run it in a version of Python < 3.9 (I tried 3.6), it generates a runtime exception on the return type annotation.

def main() -> OrderedDict[str, str]:
    x: OrderedDict[str, str] = OrderedDict()
    print(x)
    return x

    def main() -> OrderedDict[str, str]:
TypeError: 'type' object is not subscriptable

[erictraut]

I've updated the logic to differentiate between cases where this will cause runtime exceptions (parameter annotations, return type annotations, type aliases, and other non-annotation expressions) and cases where it will not (PEP 526-style variable type annotations). This change will be in the next release of Pylance.

Thanks for reporting this issue.

[jakebailey]

This issue has been fixed in version 2021.4.1, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202141-14-april-2021