Only lower if clean_key is instance of str

[vmatt]

Fix AttributeError when group_by key is None with ignore_string_case

Related Issue

Fixes #503

Description

This PR fixes a bug where using group_by with a nullable key raises an AttributeError when ignore_string_case=True. The error occurs because the code attempts to call .lower() on None values.

Changes

• Modified the string case conversion logic in diff.py to check if the key is a string before applying .lower()
• Added type checking using isinstance(clean_key, str) to safely handle None values
• +1: Black formatting was automatically for tests/test_diff_text.py, @seperman let me know if it's an issue and I should revert.

Before

if self.ignore_string_case:
    clean_key = clean_key.lower()

After

if self.ignore_string_case and isinstance(clean_key, str):
    clean_key = clean_key.lower()

Testing

Added test case:

dict1 = [{'txt_field': 'FULL', 'group_id': None}]
dict2 = [{'txt_field': 'FULL', 'group_id': 'a'}]
diff = DeepDiff(
    dict1,
    dict2,
    ignore_order=True,
    group_by='group_id',
    ignore_string_case=True
)

Checklist

• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes

[vmatt]

hey @seperman, anything needed on my side to move this forward?

[seperman]

@vmatt Thanks for making the PR!
Sorry I was busy in the past couple of weeks. Can you please not include the styling changes (Black formatting) in the PR?
Styling changes should be done in separate PRs that don't touch code complexity. Also, we are using Ruff, not Black and I will make a separate PR to reformat the whole package. Thanks!

[vmatt]

@seperman, No worries, thanks for the feedback, I removed black formatting.

[seperman]

Thanks @vmatt !

[seperman]

LGTM!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.70%. Comparing base (6d8a4c7) to head (85adbd2).
Report is 29 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #504   +/-   ##
=======================================
  Coverage   96.70%   96.70%           
=======================================
  Files          14       14           
  Lines        3946     3946           
=======================================
  Hits         3816     3816           
  Misses        130      130           

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