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`assert_type` fails to fail for `NoReturn` · Issue #16319 · python/mypy · GitHub

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assert_type fails to fail for NoReturn #16319

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assert_type fails to fail for NoReturn#16319
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bugmypy got something wrong

Description

ghost
opened on Oct 23, 2023

Bug Report

In a case that mypy deems impossible (i.e. type narrowed to NoReturn), it's currently impossible to cause assert_type to fail.

Similarly, reveal_type has no effect in the case of NoReturn.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.11&gist=10400960688da44ab22b58df6c297f50

Expected Behavior

Three assert_type failures.

Actual Behavior

Only one assert_type failure.

Activity

  1. ghost added
    bugmypy got something wrong
    on Oct 23, 2023

  2. sobolevn commented on Oct 24, 2023

    sobolevn
    on Oct 24, 2023
    Member

    This happens because mypy does not execute code for Literal[False] branches in if.

    Check this:

    """All three of these assert_type should fail, but only one does, currently."""
import typing_extensions as t

def f(x: int | bool):
    if isinstance(x, str):
        reveal_type(x)  # not executed

  3. sobolevn commented on Oct 24, 2023

    sobolevn
    on Oct 24, 2023
    Member

    This does not look like a bug to me.

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