Python 3.6.5 has trouble on folding email messages: Traceback (most recent call last): File "emailtest.py", line 7, in <module> policy.fold("Subject", msg["Subject"]) File "/usr/lib/python3.6/email/policy.py", line 183, in fold return self._fold(name, value, refold_binary=True) File "/usr/lib/python3.6/email/policy.py", line 205, in _fold return value.fold(policy=self) File "/usr/lib/python3.6/email/headerregistry.py", line 258, in fold return header.fold(policy=policy) File "/usr/lib/python3.6/email/_header_value_parser.py", line 144, in fold return _refold_parse_tree(self, policy=policy) File "/usr/lib/python3.6/email/_header_value_parser.py", line 2651, in _refold_parse_tree part.ew_combine_allowed, charset) File "/usr/lib/python3.6/email/_header_value_parser.py", line 2728, in _fold_as_ew first_part = to_encode[:text_space] TypeError: slice indices must be integers or None or have an __index__ method The message has non-ascii characters in header and set max_line_length=0, regardless length of the header. Here is the code to reproduce. from email.message import EmailMessage from email.policy import default policy = default.clone(max_line_length=0) msg = EmailMessage() msg["Subject"] = "á" policy.fold("Subject", msg["Subject"]) I first found this issue on Maildir.add, which saves the message to a file without word wrap.

Oh, interesting. I could argue that that's a missing feature in Python's slice handling. The value of text_space at that point is '+inf', and I obviously incorrectly assumed that slicing would treat that as if it were [:]. The counter argument, of course, is that inf is a float not an integer. I suppose we'll have to use sys.maxsize instead.

See this issue as a duplicata of this one: https://bugs.python.org/issue35985

 New changeset feac6cd7753425fba006e97e2d9b74a0c0c75894 by R. David Murray (Abhilash Raj) in branch 'master': bpo-33524: Fix the folding of email header when max_line_length is 0 or None (#13391) https://github.com/python/cpython/commit/feac6cd7753425fba006e97e2d9b74a0c0c75894 

 New changeset 5386aaf07835889e90fb33e95b6d37197f8cfea0 by Miss Islington (bot) in branch '3.7': bpo-33524: Fix the folding of email header when max_line_length is 0 or None (GH-13391) https://github.com/python/cpython/commit/5386aaf07835889e90fb33e95b6d37197f8cfea0 

Thank you, @licht-t for the original patch and @maxking for the rebase. Also, thank you, @r.david.murray for the review and merge.