email: IndexError when folding address headers containing empty comments #159621

Bug report

Minimal reproduce:

from email.parser import Parser
from email import policy

value = "a" * 50 + "@example.com,()b@example.com"
m = Parser(policy=policy.default).parsestr(f"To: {value}\r\n\r\nx")

assert m["To"].defects == ()
m.as_string()  # IndexError

As shown above, when there is an email valu that is over the policy's folding limit it will trigger split and a check about leading folding whitespace by recursively inspect the childs.

def startswith_fws(self):
    return self[0].startswith_fws()

When the comment is empty the token list is empty, so self[0] become index error.

Note there is a historical issue: #71445 that hit similar issue about startswith_fws() reject to working.

This issue have been reproduced in python 3.12, 3.13 and main branch.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-159621: Handle empty lists in TokenList.startswith_fws #159622