gh-145824: email.utils: replace AttributeError with TypeError in collapse_rfc2231_value for non-string non-3-tuple input

[stefanzetzsche]

Issue

collapse_rfc2231_value falls through to unquote(value) for tuples whose length is not 3, but unquote expects a string, raising AttributeError.

Reproducer

from email.utils import collapse_rfc2231_value
collapse_rfc2231_value(('a', 'b'))  # AttributeError: 'tuple' object has no attribute 'startswith'

Fix

1. In Lib/email/utils.py, raise TypeError for values that are neither a string nor a 3-tuple:

if not isinstance(value, tuple) or len(value) != 3:
    if not isinstance(value, str):
        raise TypeError(
            f"expected str or 3-tuple, got {type(value).__name__}"
        )
    return unquote(value)

2. In Doc/library/email.utils.rst, change "not a tuple" to "not a 3-tuple" to match the code's actual check.

Tests

test_collapse_rfc2231_value_non_3_tuple in TestRFC2231 (Lib/test/test_email/test_email.py) verifies that:

• Non-string, non-3-tuple values (empty tuple, 1-tuple, 2-tuple, 4-tuple, int, None) raise TypeError
• A 3-tuple ('us-ascii', 'en', 'hello') decodes correctly
• A plain quoted string '"hello"' passes through unquote

• Issue: email.utils.collapse_rfc2231_value raises AttributeError on non-3-tuples #145824

[picnixz]

I don't think we should do anything here. First, it's not a crash but rather a GIGO case (garbage in, garbage out). The documentation says:

For convenience, if the value passed to collapse_rfc2231_value() is not a tuple, it should be a string and it is returned unquoted.

So returning the stringification of the tuple may not necessarily be correct. Instead, a TypeError should be raised if AttributeError is too cryptic IMO.

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[stefanzetzsche]

Thanks @picnixz, I have made the requested changes; please review again. See also #145824 (comment).