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