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According to the documentation, HTTPSConnection.request() should accept arbitrary bytes-like objects, but this is not the case. Currently (since Issue 23756), a "bytes-like object" is defined to be anything that works with Python’s buffer API, as long as it is C-contiguous. These objects can be passed to socket.sendall():
>>> byteslike = (ctypes.c_ubyte * 6).from_buffer_copy(b"DATA\r\n")
>>> s = socket.create_connection(("localhost", 80))
>>> s.sendall(byteslike) # Server receives b"DATA\r\n"
This is not explicitly documented for socket objects. But since Issue 23539 (3.4+), HTTPConnection.request() does document support for bytes-like objects:
>>> h = HTTPConnection("localhost", 80)
>>> # Send Content-Length: 6 and body b"DATA\r\n"
>>> h.request("POST", "/", body=byteslike)
On its own, there is no problem with Python relying on its own undocumented behaviour. But Python’s "ssl" module does not support arbitrary bytes-like objects, and as a result neither does HTTPSConnection:
>>> s = ssl.wrap_socket(socket.create_connection(("localhost", 443)))
>>> s.sendall(byteslike)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/lib/python3.5/ssl.py", line 886, in sendall
 v = self.send(data[count:])
 File "/usr/lib/python3.5/ssl.py", line 856, in send
 return self._sslobj.write(data)
 File "/usr/lib/python3.5/ssl.py", line 581, in write
 return self._sslobj.write(data)
TypeError: a bytes-like object is required, not 'list'
>>> c = ssl.create_default_context(cafile="/lib/python3.5/test/keycert.pem")
>>> h = HTTPSConnection("localhost", 443, context=c)
>>> h.request("POST", "/", body=byteslike)
Traceback (most recent call last):
 File "/usr/lib/python3.5/http/client.py", line 885, in send
 self.sock.sendall(data)
 File "/usr/lib/python3.5/ssl.py", line 886, in sendall
 v = self.send(data[count:])
 File "/usr/lib/python3.5/ssl.py", line 856, in send
 return self._sslobj.write(data)
 File "/usr/lib/python3.5/ssl.py", line 581, in write
 return self._sslobj.write(data)
TypeError: a bytes-like object is required, not 'list'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/lib/python3.5/http/client.py", line 1083, in request
 self._send_request(method, url, body, headers)
 File "/usr/lib/python3.5/http/client.py", line 1128, in _send_request
 self.endheaders(body)
 File "/usr/lib/python3.5/http/client.py", line 1079, in endheaders
 self._send_output(message_body)
 File "/usr/lib/python3.5/http/client.py", line 913, in _send_output
 self.send(message_body)
 File "/usr/lib/python3.5/http/client.py", line 892, in send
 "or an iterable, got %r" % type(data))
TypeError: data should be a bytes-like object or an iterable, got <class '__main__.c_ubyte_Array_6'>
This could be fixed in the implementation of SSLSocket.sendall(). But I am not sure if that is an appropriate change for 3.5. Another option would be to adjust the documentation of HTTP(S)Connection in 3.5, either not mentioning bytes-like objects at all, or clarifying that they don’t work with SSL.

New changeset 888bbdc192ec4db888a294ef758cf5510442dc9a by Christian Heimes in branch 'master':
bpo-27340: Use memoryview in SSLSocket.sendall() (#3384)
https://github.com/python/cpython/commit/888bbdc192ec4db888a294ef758cf5510442dc9a

New changeset 9423f5d68874ff2e500cbe072c25f883cf754be8 by Christian Heimes in branch '3.6':
[3.6] bpo-27340: Use memoryview in SSLSocket.sendall() (GH-3384) (#3434)
https://github.com/python/cpython/commit/9423f5d68874ff2e500cbe072c25f883cf754be8

In Python 3.6 and master, SSLSocket.sendall() now accepts any bytes-like object that can be cast to bytes. Since it uses a sliced memoryview instead of sliced bytes, it avoids extra copies of the payload, too.