[SulimanAbdulrazzaq]

Description

The blackd docs say an invalid value in any of the request headers gets an HTTP 400 response that names the problematic header. X-Python-Variant breaks that for three kinds of value:

• an empty value;
• a bare py;
• a list with an empty entry, such as a trailing comma (3.6,) or a doubled comma (py3.6,,py3.7).

For each entry, parse_python_variant_header strips an optional py prefix and then reads version[0] outside its try block. An empty entry therefore raises IndexError, which escapes the header validation, so the handler logs a traceback and answers with HTTP 500 and the body string index out of range.

The fix rejects an empty entry with the same InvalidVariantHeader the function already raises for other malformed values. The request now gets the documented 400: Invalid value for X-Python-Variant: expected e.g. '3.7', 'py3.5'.

The four values above are added to test_blackd_invalid_python_variant. The test fails with 500 != 400 before the change and passes after it.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy? (N/A, this change is in blackd only)
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation? (N/A, this makes blackd match its existing docs)

[github-actions]

diff-shades results comparing this PR (c167f05) to main (2b96e28):

--preview style: no changes

--stable style: no changes

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What is this? | Workflow run | diff-shades documentation

[cobaltt7]

Thanks!