Steps to Reproduce
==================
>>> from email.mime.multipart import MIMEMultipart
>>> from email.mime.text import MIMEText
>>> multipart = MIMEMultipart()
>>> multipart.set_charset('UTF-8')
>>> text = MIMEText("sample text")
>>> multipart.attach(text)
>>> print multipart.as_string()
MIME-Version: 1.0
Content-Type: multipart/mixed; charset="utf-8";
        boundary="===============0973828728=="
Content-Transfer-Encoding: base64

--===============0973828728==
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

sample text
--===============0973828728==--
>>> multipart = MIMEMultipart()
>>> multipart.attach(text)
>>> multipart.set_charset('UTF-8')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/
email/message.py", line 262, in set_charset
    self._payload = charset.body_encode(self._payload)
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/
email/charset.py", line 384, in body_encode
    return email.base64mime.body_encode(s)
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/
email/base64mime.py", line 148, in encode
    enc = b2a_base64(s[i:i + max_unencoded])
TypeError: b2a_base64() argument 1 must be string or read-only buffer, 
not list

Explanation
===========
The first example above demonstrates that if you call set_charset('UTF-
8') on a MIMEMultipart instance before adding child parts then it is 
possible to generate a multipart/* message with an illegal Content-
Transfer-Encoding as specified by RFC 2045[1] "If an entity is
   of type "multipart" the Content-Transfer-Encoding is not permitted to
   have any value other than "7bit", "8bit" or "binary"."

In the second example, I demonstrate that if you try and call 
set_charset after adding child parts, the code exceptions. The user 
should at least be provided with a more targeted exception.

Notes
=====
Where should this be fixed? The smallest fix would be to add a check to 
set_charset to see if it is dealing with with a multipart message but 
as I express in issue1822 I feel the better design would be to move 
this subtype specific logic into the appropriate subclass.

Again, this is something I'm willing to work on in next saturday's bug 
day if I can get some feedback on my architectural concerns.

[1] http://tools.ietf.org/html/rfc2045#section-6.4

I'd like to question whether anything needs to be fixed at all, i.e.
whether it is the responsibility of the email package to reject all
kinds of non-sensical data. Garbage in, garbage out.

Barry, can you take a look?

Martin,

I can almost agree with you _if_ I was setting the Content-Transfer-
Encoding myself, however I am not. I am setting the charset and the 
library chooses an appropriate Content-Transfer-Encoding to represent 
the mime part with. Currently I can't see any way other than reading 
the source or writing a test case (and that would require understanding 
what the email.mime module was doing "under the hood") for a developer 
to find out which Content-Transfer-Encoding was going to be used.

Also, just from a usability point of view I would expect that creating 
an invalid mime part would be a little more difficult. Especially 
considering the fix should be as small as adding "if not encoding in 
valid encodings: raise SensibleException".

Please provide an unit test which verifies the bug and a fix for the bug.

You're right that this should probably be fixed in the subclass, but you
also have to remember that the parser generally doesn't create subclass
instances.  It only creates instances of Message.  As long as you can
make it work properly with the parser and generator, I'm okay with
overriding set_charset() in the subclass to do the right thing.

I'm beginning to realise this is slightly bigger than I first thought
 ;-)

Trying to make a nice test case for this issue, I thought it would be a 
good idea for the parser to register a defect for invalid content-
transfer-encoding so I can test against that in the test case rather 
than fragile substring tests. Unfortunately the parser code isn't the 
easiest code to get your head around on a first look.

Run out of time to look at this today. In order to write a nice test 
case for this issue I need the parser to notice this error in messages. 
I've filed issue1874 for the parser not reporting the invalid cte in 
the msg.defects