Add test for incorrect mutation error

[iscai-msft]

What do these changes do?

I'm reproing the pypy issue I found here #620

Are there changes in behavior for the user?

No

Related issue number

620

Checklist

• [x ] I think the code is well written
• [ x] Unit tests for the changes exist
• [ x] Documentation reflects the changes
• [x ] Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[codecov]

Codecov Report

Merging #627 (f90225b) into master (9dde0bd) will decrease coverage by 0.60%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #627      +/-   ##
==========================================
- Coverage   93.78%   93.18%   -0.61%     
==========================================
  Files           5        5              
  Lines         499      499              
==========================================
- Hits          468      465       -3     
- Misses         31       34       +3     

Flag	Coverage Δ
unit	93.18% <ø> (-0.61%)	⬇️

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Impacted Files	Coverage Δ
multidict/_compat.py	80.00% <0.00%> (-20.00%)	⬇️
multidict/__init__.py	88.88% <0.00%> (-11.12%)	⬇️

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[Dreamsorcerer]

Probably needs to go in test_mutable_multidict.py, as half the test iterations are failing due to immutable classes being used.

However, it looks like this test fails on all platforms, not just pypy.

Of the 4 iterations, only the multidict._multidict-CIMultiDict class actually passes and allows this behaviour. I see that this one is skipped on pypy, so that class is probably not supported on pypy. But, on all platforms the multidict._multidict_py-CIMultiDict version fails here.

Not sure what the correct behaviour is, but it looks like the C optimised version allows this mutation, while the python version does not. Presumably the C optimised version is used by default on CPython, but is not supported on pypy.

[Dreamsorcerer]

A regular dict does support this behaviour:

>>> d = {"a": "123, 456", "b": "789"}
>>> before_mutation_items = d.items()
>>> d["c"] = "000"
>>> list(before_mutation_items)
[('a', '123, 456'), ('b', '789'), ('c', '000')]

So, I'd say the Python version of multidict should probably support this as well.

[asvetlov]

Thanks a lot for the report and test.
#628 should fix the bug