Improve the "Argument must be a mapping" error message

[purna135]

Description

Fixes #12070

Improved the error message "error: Argument after ** must be a mapping" when there is a mismatch types

[JelleZijlstra]

Could you add a test case where this makes a difference? I'd grep for existing test cases that use this message, and add another one that would be different without this PR.

[purna135]

Hi @JelleZijlstra, I'm completely new to this, so I'm a little perplexed right now.
I made the following changes to the check-kwargs.test file:

[case testInvalidTypeForKeywordVarArg]
from typing import Dict
from typing import Any, Optional
def f(**kwargs: 'A') -> None: pass
d = None # type: Dict[A, A]
f(**d)         # E: Keywords must be strings
f(**A())       # E: Argument after ** must be a mapping, not "A"
class A: pass
kwargs: Optional[Any]
f(**kwargs)    # E: Argument after ** must be a mapping, not "Union[Any, None]"
[builtins fixtures/dict.pyi]

But running the pytest -n0 -k 'testInvalidTypeForKeywordVarArg' gives the below output

E   AssertionError: Unexpected type checker output (C:\Users\Purna\Downloads\OpenSource\mypy\test-data\unit\check-kwargs.test, line 352)
------------------------------------------------------------------- Captured stderr call -------------------------------------------------------------------
Expected:
  main:5: error: Keywords must be strings
  main:6: error: Argument after ** must be a mapping, not "A"
  main:9: error: Argument after ** must be a mapping, not "Union[Any, None]" (diff)
Actual:
  main:5: error: Keywords must be strings
  main:6: error: Argument after ** must be a mapping, not "A"

================================================================= short test summary info ==================================================================
FAILED mypy/test/testcheck.py::TypeCheckSuite::check-kwargs.test::testInvalidTypeForKeywordVarArg

Please guide how to solve this, Thanks

[JelleZijlstra]

You may have to turn on strict optional mode in the test case (put # flags: --strict-optional at the top of the case).

[purna135]

Thanks @JelleZijlstra, this time I got some error : )
First of all there was a Incompatible types due to d = None # type: Dict[A, A], so I removed this check temporarily to check my test.

Below is the error, what I got before this PR
Test:

[case testInvalidTypeForKeywordVarArg]
# flags: --strict-optional
from typing import Dict
from typing import Any, Optional
def f(**kwargs: 'A') -> None: pass
f(**A())       # E: Argument after ** must be a mapping, not "A"
class A: pass
kwargs: Optional[Any]
f(**kwargs)    # E: Argument after ** must be a mapping, not "Union[Any, None]"
[builtins fixtures/dict.pyi]

Output:

Expected:
  main:5: error: Argument after ** must be a mapping, not "A"
  main:8: error: Argument after ** must be a mapping, not "Union[Any, None]" (diff)
Actual:
  main:5: error: Argument after ** must be a mapping, not "A"
  main:8: error: Argument after ** must be a mapping (diff)

Alignment of first line difference:
  E: ...** must be a mapping, not "Union[Any, None]"
  A: ...** must be a mapping

After this PR, below is the output so I have to replace Union[Any, None] with Optional[Any]

Expected:
  main:5: error: Argument after ** must be a mapping, not "A"
  main:8: error: Argument after ** must be a mapping, not "Union[Any, None]" (diff)
Actual:
  main:5: error: Argument after ** must be a mapping, not "A"
  main:8: error: Argument after ** must be a mapping, not "Optional[Any]" (diff)

Alignment of first line difference:
  E: ...** must be a mapping, not "Union[Any, None]"
  A: ...** must be a mapping, not "Optional[Any]"
                                   ^

Shoul I replace the message with

self.fail(
      'Argument after ** has incompatible type {}, expected "Mapping[str, Any]"'.format(format_type(typ)),
      context, code=codes.ARG_TYPE)

[purna135]

and one more question, should I remove # flags: --strict-optional from test while commiting the changes ?

[purna135]

Hello, I'm waiting for your response; could you please direct me in the right direction?

[JelleZijlstra]

Hi, that's a lot of questions.

For Union[Any, None], just change the test output to whatever variant makes it pass. We recently merged a PR that I think affects the output here.

I'm not sure why you are suggesting a message with Mapping[str, Any] in it. I feel like that would make the message harder to read for users.

And no, the --strict-optional should stay, because otherwise the tests will fail.

[purna135]

Thanks @JelleZijlstra

--strict-optional causes the test to fail for below case with the message main:4: error: Incompatible types in assignment (expression has type "None", variable has type "Dict[A, A]")

def f(**kwargs: 'A') -> None: pass
d = None # type: Dict[A, A]
f(**d)         # E: Keywords must be strings

I am pushing the changes please have a look.

[JelleZijlstra]

Suggested change

	d = None # type: Dict[A, A]
	d = {} # type: Dict[A, A]

Does this work?

[purna135]

Yes now it's passing all the tests, Thanks : )

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JelleZijlstra]

Thanks for your contribution!