Exception when using only whitespace characters as e-mail subject

[Moritz Sichert]

Steps to reproduce:

Python 3.5.2 (default, Jun 28 2016, 08:46:01) 
[GCC 6.1.1 20160602] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import django.conf
>>> django.conf.settings.configure()
>>> from django.core import mail
>>> mail.EmailMessage(subject=' ', from_email='django@example.com').message()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/django/django/core/mail/message.py", line 311, in message
    msg['Subject'] = self.subject
  File "/home/user/django/django/core/mail/message.py", line 236, in __setitem__
    name, val = forbid_multi_line_headers(name, val, self.encoding)
  File "/home/user/django/django/core/mail/message.py", line 102, in forbid_multi_line_headers
    val = Header(val).encode()
  File "/usr/lib/python3.5/email/header.py", line 387, in encode
    value = formatter._str(linesep)
  File "/usr/lib/python3.5/email/header.py", line 423, in _str
    self.newline()
  File "/usr/lib/python3.5/email/header.py", line 435, in newline
    self._lines[-1] += str(self._current_line)
IndexError: list index out of range

This error doesn't occur when directly setting msg['Subject'] = ' ' on a plain email.message.Message. Using subjects like ' foo' or 'foo ' also works.

It might still be a bug in Python, though, as the Exception isn't raised in Django's code but in the email module while trying to encode the Subject header.

[Tim Graham]

It seems to be a regression in Python 3. I created ​http://bugs.python.org/issue27737. I don't think the issue is so critical that we need to add a workaround in Django but if someone feels otherwise and wants to submit a patch, feel free to reopen.