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I'm trying to use the email.* functions to craft HTTP POST data for file upload. Trying something like
filedata = open("data", "rb").read()
postdata = MIMEMultipart()
fileattachment = MIMEApplication(filedata, _encoder=email.encoders.encode_noop)
postdata.attach(fileattachment)
fp = BytesIO()
g = BytesGenerator(fp)
g.flatten(postdata, unixfrom=False)
fails with 
Traceback (most recent call last):
 File "./minetest.py", line 30, in <module>
 g.flatten(postdata, unixfrom=False)
 File "/usr/lib/python3.2/email/generator.py", line 91, in flatten
 self._write(msg)
 File "/usr/lib/python3.2/email/generator.py", line 137, in _write
 self._dispatch(msg)
 File "/usr/lib/python3.2/email/generator.py", line 163, in _dispatch
 meth(msg)
 File "/usr/lib/python3.2/email/generator.py", line 224, in _handle_multipart
 g.flatten(part, unixfrom=False, linesep=self._NL)
 File "/usr/lib/python3.2/email/generator.py", line 91, in flatten
 self._write(msg)
 File "/usr/lib/python3.2/email/generator.py", line 137, in _write
 self._dispatch(msg)
 File "/usr/lib/python3.2/email/generator.py", line 163, in _dispatch
 meth(msg)
 File "/usr/lib/python3.2/email/generator.py", line 192, in _handle_text
 raise TypeError('string payload expected: %s' % type(payload))
TypeError: string payload expected: <class 'bytes'>
This is because BytesGenerator._handle_text() expects str payload in which byte values that are non-printable in ASCII have been replaced by surrogates. The example above creates a bytes payload, however, for which super(BytesGenerator,self)._handle_text(msg) = Generator._handle_text(msg) throws the exception.
Note that using any email.encoders other than encode_noop does not really fit the HTTP POST bill, as those define a Content-Transfer-Encoding which HTTP does not know.
It would seem better to me to let BytesGenerator accept a bytes payload and just copy that to the output, rather than making the application encode the bytes as a string, hopefully in a way that s.encode('ascii', 'surrogateescape') can invert.
E.g., a workaround class I use now does
class FixedBytesGenerator(BytesGenerator):
 def _handle_bytes(self, msg): 
 payload = msg.get_payload()
 if payload is None:
 return
 if isinstance(payload, bytes):
 self._fp.write(payload) 
 elif isinstance(payload, str):
 super(FixedBytesGenerator,self)._handle_text(msg)
 else:
 # Payload is neither bytes not string - this can't be right
 raise TypeError('bytes or str payload expected: %s' % type(payload))
 _writeBody = _handle_bytes