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When using MIME email package you can specify "utf8" as the encoding. It
will be accepted but it is not rendered correctly in some MUA. E.g. Mac
OS X Mail.app doesn't display it properly while Google Gmail does.
It is confusing since Python itself happily understands both utf8 and utf-8.
The patch adds "utf8" as an alias to "utf-8" encoding which means user
won't need to think twice.
Test case:
from email.MIMEText import MIMEText
msg = MIMEText(u'\u043a\u0438\u0440\u0438\u043b\u0438\u0446\u0430')
msg.set_charset('utf8')
print msg.as_string()

This seems entirely reasonable, helpful, and in accord with the mapping
of ascii to us-ascii. I recommend accepting this patch or a slightly
fancier one that would also do "utf_8".
There are pobably other encoding names with the same issue of being
accepted by Python but not be understood by other email clients.
This issue also affects 2.6.1 and 2.7trunk. I haven't checked 3.x.

Has this patch been merged yet?

I tested it on python 2.5, 2.6, 2.7 trunk and 3.2 varying msg.set_charset(x) with x = 'utf8' and 'utf-8' 
Here are the results. Apparantly python 2.x had issue with Test case and 3.2 passed but I guess it is unrelated with the issue.
07:35:40 l0nwlf-MBP:~ $ python2.5
Python 2.5.4 (r254:67916, Jul 7 2009, 23:51:24) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from email.MIMEText import MIMEText
>>> msg = MIMEText(u'\u043a\u0438\u0440\u0438\u043b\u0438\u0446\u0430')
>>> msg.set_charset('utf8')
>>> print msg.as_string()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/email/message.py", line 131, in as_string
 g.flatten(self, unixfrom=unixfrom)
 File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/email/generator.py", line 84, in flatten
 self._write(msg)
 File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/email/generator.py", line 109, in _write
 self._dispatch(msg)
 File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/email/generator.py", line 135, in _dispatch
 meth(msg)
 File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/email/generator.py", line 178, in _handle_text
 self._fp.write(payload)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-7: ordinal not in range(128)
>>> msg.set_charset('utf-8')
>>> print msg.as_string()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/email/message.py", line 131, in as_string
 g.flatten(self, unixfrom=unixfrom)
 File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/email/generator.py", line 84, in flatten
 self._write(msg)
 File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/email/generator.py", line 109, in _write
 self._dispatch(msg)
 File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/email/generator.py", line 135, in _dispatch
 meth(msg)
 File "/System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/email/generator.py", line 178, in _handle_text
 self._fp.write(payload)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-7: ordinal not in range(128)
07:36:17 l0nwlf-MBP:~ $ python2.6
Python 2.6.5 (r265:79063, Apr 6 2010, 21:34:21) 
[GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from email.MIMEText import MIMEText
>>> msg = MIMEText(u'\u043a\u0438\u0440\u0438\u043b\u0438\u0446\u0430')
>>> msg.set_charset('utf8')
>>> print msg.as_string()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/email/message.py", line 135, in as_string
 g.flatten(self, unixfrom=unixfrom)
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/email/generator.py", line 84, in flatten
 self._write(msg)
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/email/generator.py", line 109, in _write
 self._dispatch(msg)
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/email/generator.py", line 135, in _dispatch
 meth(msg)
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/email/generator.py", line 178, in _handle_text
 self._fp.write(payload)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-7: ordinal not in range(128)
>>> msg.set_charset('utf-8')
>>> print msg.as_string()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/email/message.py", line 135, in as_string
 g.flatten(self, unixfrom=unixfrom)
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/email/generator.py", line 84, in flatten
 self._write(msg)
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/email/generator.py", line 109, in _write
 self._dispatch(msg)
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/email/generator.py", line 135, in _dispatch
 meth(msg)
 File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/email/generator.py", line 178, in _handle_text
 self._fp.write(payload)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-7: ordinal not in range(128)
 
07:36:37 l0nwlf-MBP:~ $ python2.7
Python 2.7a4+ (trunk:78750, Mar 7 2010, 08:09:00) 
[GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from email.MIMEText import MIMEText
 >>> msg = MIMEText(u'\u043a\u0438\u0440\u0438\u043b\u0438\u0446\u0430')
>>> msg.set_charset('utf8')
>>> print msg.as_string()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/local/lib/python2.7/email/message.py", line 135, in as_string
 g.flatten(self, unixfrom=unixfrom)
 File "/usr/local/lib/python2.7/email/generator.py", line 83, in flatten
 self._write(msg)
 File "/usr/local/lib/python2.7/email/generator.py", line 108, in _write
 self._dispatch(msg)
 File "/usr/local/lib/python2.7/email/generator.py", line 134, in _dispatch
 meth(msg)
 File "/usr/local/lib/python2.7/email/generator.py", line 180, in _handle_text
 self._fp.write(payload)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-7: ordinal not in range(128)
>>> msg.set_charset('utf-8')
>>> print msg.as_string()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/local/lib/python2.7/email/message.py", line 135, in as_string
 g.flatten(self, unixfrom=unixfrom)
 File "/usr/local/lib/python2.7/email/generator.py", line 83, in flatten
 self._write(msg)
 File "/usr/local/lib/python2.7/email/generator.py", line 108, in _write
 self._dispatch(msg)
 File "/usr/local/lib/python2.7/email/generator.py", line 134, in _dispatch
 meth(msg)
 File "/usr/local/lib/python2.7/email/generator.py", line 180, in _handle_text
 self._fp.write(payload)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-7: ordinal not in range(128)
07:37:06 l0nwlf-MBP:~ $ python3.2
Python 3.2a0 (py3k:79532, Apr 1 2010, 01:48:52) 
[GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from email.mime.text import MIMEText
>>> msg = MIMEText('\u043a\u0438\u0440\u0438\u043b\u0438\u0446\u0430')
>>> msg.set_charset('utf8')
>>> print (msg.as_string())
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="utf8"
кирилица
>>> msg.set_charset('utf-8')
>>> print (msg.as_string())
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="utf-8"
кирилица
The aliasing of 'utf-8' into 'utf8' and may be 'utf_8' seems reasonable IMO however Test Case fails.

MIMEText doesn't support unicode input. This was the reason OP Test case failed. For reference : http://bugs.python.org/issue1368247 

For various reasons the email module has a table of character sets. What might be most effective would be for the email module to look a character set name up in the codecs module and find out the cannonical name of the character set, and then look that up in its table (ie: remove the aliases table from email completely, and instead depend on codecs to resolve the cannonical name). Unfortunately the codecs module does not recognize all of the aliases used by email, nor is there necessarily any guarantee that the two modules will agree on the proper cannonical name.
The attached patch instead uses the codecs module as a fallback if the charset name does not appear in the email package's ALIASES or CHARSETS tables. It therefore makes both utf8 and utf_8 work, as well as all the other variants the codec module accepts. The unit test just tests 'utf8', since if that one works all the others should too.
I'm tentatively reclassifying this as a bug rather than a feature request, since I think it is a reasonable expectation that email would support at least the same set of encoding names that the rest of Python does.

Idea: Import the aliases mapping from codecs and extend it with email-specific aliases. Alternate idea: Add email’s names to codecs.
Side note: "charset" stands for "character encoding", not "character set". See <http://www.w3.org/International/questions/qa-what-is-encoding#what>

R. David Murray wrote:
> 
> R. David Murray <rdmurray@bitdance.com> added the comment:
> 
> For various reasons the email module has a table of character sets. What might be most effective would be for the email module to look a character set name up in the codecs module and find out the cannonical name of the character set, and then look that up in its table (ie: remove the aliases table from email completely, and instead depend on codecs to resolve the cannonical name). Unfortunately the codecs module does not recognize all of the aliases used by email, nor is there necessarily any guarantee that the two modules will agree on the proper cannonical name.
I think that the encodings package should be the only source of
valid aliases and encoding names - after all, you wouldn't be
able to process email content using names or aliases not
appearing in the encodings package tables.
If there are aliases missing, then we can add them there.
If the email packages needs different canonical names, it can apply
its own map on the canonical names returned by the encodings package.

Mark, any objection to my putting this patch in now, and then we'll fix the aliases implementation in 3.2?

R. David Murray wrote:
> 
> R. David Murray <rdmurray@bitdance.com> added the comment:
> 
> Mark, any objection to my putting this patch in now, and then we'll fix the aliases implementation in 3.2?
No. Please open a new issue targeting Python 3.2 for this.
Thanks,
-- 
Marc-Andre Lemburg
eGenix.com
________________________________________________________________________
2010年07月19日: EuroPython 2010, Birmingham, UK 44 days to go
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 Registered at Amtsgericht Duesseldorf: HRB 46611
 http://www.egenix.com/company/contact/ 

Patch committed to trunk in r81705. Leaving issue open pending porting to the other branches, but I've also opened issue 8898 to further change things so that codecs becomes the sole authority for aliases in 3.2.

David, can this issue be closed?

Yes. Benjamin merged this to py3k in r82292. If someone wants to explain to me how to cherry pick the changeset into 3.1 I'd be happy to do it, otherwise I think I'm done with this one :)