Should use fully qualified names in error messages

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
42/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
compilers

Research direction

No file, test, or entry point is named. Reproduce the reported diagnostics involving same-named classes from different modules, then trace mypy's error-message formatting and related tests. Done means diagnostics clearly distinguish the classes by their fully qualified names without obscuring the existing type information.

Written by the indexing model from the issue text.

Description

bug priority-1-normal topic-usability

When mixing classes with the same name defined in different modules, it's possible to generate error messages like this that are a little difficult to decipher:

/tmp/my.py:19: error: Argument 1 to "add_done_callback" of "Future" has incompatible type "Callable[[Future[T]], None]"; expected "Callable[[Future[T]], Any]"                                                                                 
/tmp/my.py:20: error: Argument 1 to "add_done_callback" of "Future" has incompatible type "Callable[[Arg(Future[T], 'source')], None]"; expected "Callable[[Future[T]], None]"

The actual problem was that the "Future" on one side of the error was from a different module. It's especially difficult to spot because of the unrelated superficial differences: I assumed there was some kind of significance to one side having a named Arg while the other doesn't.

I ran into this while writing something similar to asyncio.wrap_future, which converts from one library's Future class to another's, and getting the two mixed up in my type annotations.

Including the fully qualified class names would have made the error message much more obvious.

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