How to assert_never in a match statement

[Jasha10]

I'd like to call assert_never in the irrefutable branch of a match statement:

from typing_extensions import assert_never

def get_val() -> int | str:
    return 1

match get_val():
    case int():
        print("got int")
    case str():
        print("got str")
    case never:
        assert_never(never)

Calling assert_never like this (as opposed to omitting the final match case) has some benefits:

• It makes clear to humans that all possibilities are exhausted by the preceding cases, and
• It forces type checkers to verify that case never won't be reached.
• It gives an AssertionError at runtime if the case is reached due to a bug.

Pyright complains about the above usage:

$ pyright tmp.py
/home/homestar/dev/mrepo_wip/pysc/tmp.py
  /home/homestar/dev/mrepo_wip/pysc/tmp.py:11:10 - error: Pattern will never be matched for subject type "Never" (reportUnnecessaryComparison)
1 error, 0 warnings, 0 informations
$ pyright --version
pyright 1.1.310

My solution for now is to ignore this pyright error:

...
    case never:  # pyright: ignore [reportUnnecessaryComparison]
        assert_never(never)

Is there a better way to assert that the match arms are exhaustive (i.e. one that doesn't trigger a pyright error)? If not, @msfterictraut, would you be amenable to a feature request to ease pyright's error reporting for this use-case?

[Jasha10]

I found that the Python typing team has published example code that endorses use of assert_never in the last arm of a match statement:
https://typing.readthedocs.io/en/latest/source/unreachable.html#assert-never-and-exhaustiveness-checking
Given the official endorsement of this pattern, I will close this discussion and instead open a feature request.

Edit:
So it turns out that pyright only rejects this pattern if typeCheckingMode is set to "strict". Perhaps I was too quick to open this discussion. Mea culpa.

[erictraut]

This is working as intended. Pyright already provides a way to detect non-exhaustive matches using the reportMatchNotExhausted, so there's no need to use assert_never.

If you include the unnecessary case statement and enable the reportUnnecessaryComparison rule, pyright will tell you that the case statement isn't needed.

So your options are:

1. Enable reportMatchNotExhausted (which is on by default in strict mode) and do not add the extraneous case statement. (This is the solution I recommend.)
2. Leave the extraneous case statement in place and do one of the following:

• Disable the reportUnnecessaryComparison rule (which is off by default in "basic" type checking mode but on by default in "strict" mode.
• Use a # type: ignore or # pyright: ignore comment to suppress the error.