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.