RFC5322[0] in section 2.1.1 mandates that the line cannot be longer than 998 characters and should not be longer than 78 characters (excluding CRLF). When we use raw_data_manager (default for EmailPolicy, EmailMessage) it does the correct thing as long as the message contains characters outside of 7bit US-ASCII set - base64 or qp Content-Transfer-Encoding which respects the line wrapping at 78 characters.

However if our message is limited the characters from the 7bit US-ASCII set no transfer encoding is applied, and such messages can easily go beyond 78 or even 998 characters.


[0]: https://tools.ietf.org/html/rfc5322.html#section-2.1.1

The PR looks good to me, but I describe the change differently.  I'm not sure how I missed this in the original implementation, since I obviously checked it for the 8bit case.  Too long ago to remember :)

New changeset 6f2f475d5a2cd7675dce844f3af436ba919ef92b by Arkadiusz Hiler in branch 'master':
bpo-40597: email: Use CTE if lines are longer than max_line_length consistently (gh-20038)
https://github.com/python/cpython/commit/6f2f475d5a2cd7675dce844f3af436ba919ef92b

Thanks, Arkadiusz.

New changeset c1f1ddf30a595c2bfa3c06e54fb03fa212cd28b5 by Miss Islington (bot) in branch '3.8':
bpo-40597: email: Use CTE if lines are longer than max_line_length consistently (gh-20038) (gh-20084)
https://github.com/python/cpython/commit/c1f1ddf30a595c2bfa3c06e54fb03fa212cd28b5

With the fix in PR 20038, committed at https://github.com/python/cpython/commit/6f2f475d5a2cd7675dce844f3af436ba919ef92b it is no longer possible to set_content(''). Attempts to do so produce the following
```
  File "/var/MM/3/hk_39/hyperkitty/.tox/py39-django30/lib/python3.9/site-packages/django_mailman3/lib/scrub.py", line 95, in _get_all_attachments
    part.set_content('')
  File "/usr/local/lib/python3.9/email/message.py", line 1171, in set_content
    super().set_content(*args, **kw)
  File "/usr/local/lib/python3.9/email/message.py", line 1101, in set_content
    content_manager.set_content(self, *args, **kw)
  File "/usr/local/lib/python3.9/email/contentmanager.py", line 37, in set_content
    handler(msg, obj, *args, **kw)
  File "/usr/local/lib/python3.9/email/contentmanager.py", line 185, in set_text_content
    cte, payload = _encode_text(string, charset, cte, msg.policy)
  File "/usr/local/lib/python3.9/email/contentmanager.py", line 149, in _encode_text
    if max(len(x) for x in lines) <= policy.max_line_length:
ValueError: max() arg is an empty sequence
```

New changeset 4fa61a7732923f92de0f7830c12da48c4cec937f by Mark Sapiro in branch 'master':
bpo-40597: Allow email.contextmanager set_content() to set a null string. (GH-20542)
https://github.com/python/cpython/commit/4fa61a7732923f92de0f7830c12da48c4cec937f

New changeset c1c50345933efca42169f03d79ff4fe3d9c06bdc by Miss Islington (bot) in branch '3.8':
bpo-40597: Allow email.contextmanager set_content() to set a null string. (GH-20542)
https://github.com/python/cpython/commit/c1c50345933efca42169f03d79ff4fe3d9c06bdc

New changeset e68978978f49cfc489fa0f888281ce84460818c0 by Miss Islington (bot) in branch '3.9':
bpo-40597: Allow email.contextmanager set_content() to set a null string. (GH-20542)
https://github.com/python/cpython/commit/e68978978f49cfc489fa0f888281ce84460818c0

Closing this since the PRs for the behavior change reported by Mike and BPO-41206 is now merged too.

Thanks Mark!