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Support for interleaved errors

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
30/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
testing

Research direction

Start by reviewing the linked implementation in tests/mypy_helpers.py and its usage in tests/test_plugin.py, then inspect assertion rewriting in tests/init.py. Compare the proposed interleaved-error format with the current test expectations and determine the required plugin behavior and test coverage before deciding whether the approach can be supported.

Written by the indexing model from the issue text.

Description

reporting

Here's what I came up with while trying to replicate mypy's own testcases, which interleave the expected errors in the source code like so:
https://github.com/ikonst/pynamodb-mypy/blob/9453404f576c4ead04749d61eb041e10bec13229/tests/test_plugin.py

The implementation is here:
https://github.com/ikonst/pynamodb-mypy/blob/9453404f576c4ead04749d61eb041e10bec13229/tests/mypy_helpers.py

In a failure case, the failed assertion output will look like:

>       assert actual == expected
E       assert 'from pynamod...ibute "lower"' == 'from pynamodb..._attr.lower()'
E           from pynamodb.attributes import UnicodeAttribute
E           from pynamodb.models import Model
E
E           class MyModel(Model):
E               my_attr = UnicodeAttribute(null=True)
E
E           reveal_type(MyModel.my_attr)  # E: Revealed type is 'pynamodb.attributes._NullableAttribute[pynamodb.attributes.UnicodeAttribute, builtins.str]'
E           reveal_type(MyModel().my_attr)  # E: Revealed type is 'Union[builtins.str*, None]'
E         - MyModel().my_attr.lower()  # E: Item "None" of "Optional[str]" has no attribute "lower"
E         + MyModel().my_attr.lower()

Of course, this requires assertion rewriting, which would've been done in the pytest plugin normally:
https://github.com/ikonst/pynamodb-mypy/blob/9453404f576c4ead04749d61eb041e10bec13229/tests/__init__.py#L3

There are two advantages of interleaving the errors in the test code:

  1. changing the test code does not necessitate updating of line numbers
  2. updating the test code is easier (by diffing the program against the "expected program", i.e. data-driven testing)

Have you considered allowing a similar approach?

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