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The MIMEText class of the email package in Python3 requires that a character set be specified in order for the resulting email to be valid. If no character set is specified, it currently assumes ascii but puts a unicode payload in the message. Because someone might actually be making use of this bug, I'm not going to backport a fix, but I do want to make utf8 the default if there are non-ascii characters in the input, in a similar fashion to how it is done for headers (well, now that *that* bug has been fixed).

Patch to default character set to utf8 if non-ascii characters are found in the input string. Please let me know if this is what you had in mind.

Pretty close. I'd do the check for us_ascii first, and only do the encode test/switch to utf-8 if that's the charset. The reason is that that if a charset has been specified, we don't waste time doing an unnecessary encoding (and the ascii codec is very fast, which you can't say about all the codecs).
Now, what would be *really* nice is to also try latin-1 before falling back to utf-8, but I wouldn't want to make that the default behavior for performance reasons. I'm planning to add support for that at some point, but I haven't decided exactly how (policy setting? New optional setting in the alias structure?)
There seem to be unrelated changes to torture_test in your patch?

Understood. Please take a look at the updated patch. Also, removed the unrelated diffs that somehow were hanging around in my hg mq.

That looks good.

New changeset bfd1ba2bbaf8 by R David Murray in branch 'default':
#14380: Have MIMEText defaults to utf-8 when passed non-ASCII unicode
http://hg.python.org/cpython/rev/bfd1ba2bbaf8 

See also issue 7304. It was niggling at the back of my brain, and I finally managed to dig it up. Fixing that is much more complex than fixing this (because set_charset is a *very* strange method), so I committed this patch in case we don't manage to fix 7304 before 3.3 is ready, but with a comment about removing it when 7304 is fixed.
Thanks, Jeff.

New changeset 9ceac471bd8c by R David Murray in branch 'default':
#14380: Make actual default match docs, fix __init__ order.
http://hg.python.org/cpython/rev/9ceac471bd8c 

Maybe it'll be better to use 'latin-1' charset for latin-1 texts?
Something like this:
if _charset == 'us-ascii':
 try:
 _text.encode(_charset)
 except UnicodeEncodeError:
 try:
 _text.encode('latin-1')
 except UnicodeEncodeError:
 _charset = 'utf-8'
 else:
 _charset = 'latin-1'
This will make messages in most latin languages use quoted-printable encoding.

I do plan to add something like that at some point. You could open a new issue for it if you like, and propose a formal patch.