Fixes #3427

[MIXgleb]

I have made things!

• Adds WPS481: forbids using complex slice indexes, Forbid complex grammar in slice #3427 . Only the Name, Constant, Attribute, Unary and Bin operators with simple expressions inside parentheses are allowed.

Correct:

array[3:7]
array[:-(x + 3)]
array[::-index.step]
array[start_index:end_index]
array[index.start:index.end]
array[Slice.start:]
array[(x // y):]

Wrong:

array[my_dict["start_index"]:my_list[-1]]
array[start_index():index.end()]
array[Slice().start:]
array[-(-x + 1):]
array[(x * (y + 2)):]
array[(x + y + z):]

Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
• I have created at least one test case for the changes I have made
• I have updated the documentation for the changes I have made
• I have added my changes to the CHANGELOG.md

Related issues

• Closes Forbid complex grammar in slice #3427

The new check WPS481 found a complex slice in the wemake_python_styleguide/violations/base.py file. It has been updated and moved to a separate variable.

end_index = len(text) - len(lstripped_text)
leading_whitespaces = text[:end_index]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (71eebb4) to head (b905e49).
⚠️ Report is 72 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #3433   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          362       363    +1     
  Lines        11987     12042   +55     
  Branches       821       831   +10     
=========================================
+ Hits         11987     12042   +55     

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[sobolevn]

Thanks for the PR! As I said in the issue, I am still not sure about it. But, with this PR it would be easier for me to analyze existing code.

[MIXgleb]

I also have an idea to allow the use of certain functions, such as len and count.

This will be stored in a global list for user customization:
FUNCTION_IN_SLICE_WHITELIST: Final = frozenset({'len', 'count'})