Improve the "Argument must be a mapping" error message

[DevilXD]

Feature

This could also be classified as a bug report, but it's also a feature request. Consider code like this:

from typing import Any, Optional

def test(**kwargs):
    ...

kwargs: Optional[Any]
test(**kwargs)

Now, this is obviously wrong on purpose, and in latest MyPy, results in this error message:

error: Argument after ** must be a mapping

The problem with this message is that, it doesn't say what exactly is wrong, unlike when you normally mismatch types where it says: Argument 1 to "test" has incompatible type "Union[int, str]"; expected "bool", for example. In my case, I had a variable with the same name inferring Optional[Any] higher up in the code, and tracking down the root cause took me good 10 minutes,

Pitch

Even though **kwargs usage like this is uncommon, having a similarly improved message would help with debugging why this may be happening. Something like: Argument after ** to "test" has incompatible type "Union[Any, None]", expected "Mapping[str, Any]" should help a lot.

[JelleZijlstra]

Yes, we should improve this. PR welcome!

[himanshi-2602]

@JelleZijlstra
I would like to work on this issue.
I am a beginner but really interested in open source.

[JelleZijlstra]

Go for it! I haven't looked at the code for this error, but here's what I would do:

• Find the place where the must be a mapping error is raised. Start by grepping for this string, then keep grepping to look for the code where we actually perform the check.
• Find out how to get additional information to make the error more informative. Look for examples where we already give a more informative error, e.g. when there's an argument type mismatch.
• Update the code you found to give the more detailed error, and update the test suite.

[himanshi-2602]

@JelleZijlstra
Thanks for the response.

[purna135]

Hi @JelleZijlstra, I am new to open source and really interested to contribute my favorite language i.e python, may I work on this issue or could you please suggest any other beginner-friendly issue to start with. Thanks!!

[JelleZijlstra]

This is a pretty good one to start! Everyone reading this should feel free to go ahead, start coding, and submit a PR.

[purna135]

Thanks @JelleZijlstra

[MihirHundiwala]

Hello, even i am a beginner to open source contribution, found this good-first-issue, so i have managed to find the file where we raise the issue 'Argument after ** must be a mapping' but i don't know what grepping is, how it is done, also how can i debug the process step by step, i know the python lib 'pdb' for debugging but how do i call it with mypy