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The email.generator.Generator class does not work correctly message 
objects created with binary data (MIMEImage, MIMEAudio, MIMEApplication, 
etc.). For example:
>>> from email.mime.image import MIMEImage
>>> data = open("IMG.jpg","rb").read()
>>> m = MIMEImage(data,'jpeg')
>>> s = m.as_string()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/tmp/lib/python3.0/email/message.py", line 136, in as_string
 g.flatten(self, unixfrom=unixfrom)
 File "/tmp/lib/python3.0/email/generator.py", line 76, in flatten
 self._write(msg)
 File "/tmp/lib/python3.0/email/generator.py", line 101, in _write
 self._dispatch(msg)
 File "/tmp/lib/python3.0/email/generator.py", line 127, in _dispatch
 meth(msg)
 File "/tmp/lib/python3.0/email/generator.py", line 155, in 
_handle_text
 raise TypeError('string payload expected: %s' % type(payload))
TypeError: string payload expected: <class 'bytes'>
>>> 
The source of the problem is rather complicated, but here is the gist of 
it.
1. Classes such as MIMEAudio and MIMEImage accept raw binary data as 
input. This data is going to be in the form of bytes.
2. These classes immediately encode the data using a base64 encoder. 
This encoder uses the library function base64.b64encode().
3. base64.b64encode() takes a byte string as input and returns a byte 
string as output. So, even after encoding, the payload of the message 
is of type 'bytes'
4. When messages are generated, the method Generator._dispatch() is 
used. It looks at the MIME main type and subtype and tries to dispatch 
message processing to a handler method of the form 
'_handle_type_subtype'. If it can't find such a handler, it defaults 
to a method _writeBody(). For image and audio types, this is what 
happens. 
5. _writeBody() is an alias for _handle_text().
6. _handle_text() crashes because it's not expecting a payload of type 
'bytes'.
Suggested fix:
I think the library function base64.b64encode() should return a string, 
not bytes. The whole point of base64 encoding is to take binary data 
and encode it into characters safe for inclusion in text strings. 
Other fixes:
Modify the Generator class in email.generator to properly detect bytes 
and use a different _handle function for it. For instance, maybe add a 
_handle_binary() method.

> I think the library function base64.b64encode() should return 
> a string, not bytes.
Yes, in the email module, the payload is an unicode string, not a 
bytes string. We have to be able to concatenate headers 
(eg. "Content-Type: image/fish\nMIME-Version: 
1.0\nContent-Transfer-Encoding: base64\n") and encoded data 
(eg. "R0lGO").
Attached patch implements this fix: encode_base64() returns str (and 
not bytes). The patchs fixes the unit tests and adds a new regression 
test for MIMEImage.as_string().

Hello,
This patch has never been commited. I tested today with the 3.1 branch (and checked in the lib code). Is there a better way to attach images in an email ?
Thanks in advance for your help,
Regards,
Stac

Hi,
I think the bug is also present in the function encode_quopri which should look like this :
def encode_quopri(msg):
 """Encode the message's payload in quoted-printable.
 Also, add an appropriate Content-Transfer-Encoding header.
 """
 orig = msg.get_payload()
 encdata = _qencode(orig)
 data = str(encdata, "ASCII")
 msg.set_payload(data)
 msg['Content-Transfer-Encoding'] = 'quoted-printable'

I wrote a patch for base64.b64encode() to accept str (str is encoded to utf-8): patch attached to #4768. It should fix this issue, but we can add the tests of email_base64_bytes.patch.

Attaching patch from reported duplicate issue8896.

Note that my patch is roughly the same as the original posted by haypo.

Yes, but yours was better formatted, so I used it :) Thanks for the patch. Applied in r81685 to py3k, and r81686.

@garazi111: if you have an example where quopri fails, please open a new issue for it. I suspect you are right that there is a problem there.

For the record, encode_quopri was fixed in #14360.