RFC 7230 3.3.2 states "A sender MUST NOT send a Content-Length header field in any message that contains a Transfer-Encoding header field."

import httplib
http = httplib.HTTPSConnection('google.com')
http.set_debuglevel(1)
http.request("POST", '/', None, {'Content-Type': 'text/plain','Transfer-Encoding': 'chunked'})
send: 'POST / HTTP/1.1\r\nHost: google.com\r\nAccept-Encoding: identity\r\nContent-Length: 0\r\nTransfer-Encoding: chunked\r\nContent-Type: text/plain\r\n\r\n'

Includes both content-length and transfer-encoding. With this change:

import httplib
http = httplib.HTTPSConnection('google.com')
http.set_debuglevel(1)
http.request("POST", '/', None, {'Content-Type': 'text/plain','Transfer-Encoding': 'chunked'})
send: 'POST / HTTP/1.1\r\nHost: google.com\r\nAccept-Encoding: identity\r\nTransfer-Encoding: chunked\r\nContent-Type: text/plain\r\n\r\n'

Includes only transfer-encoding.

https://bugs.python.org/issue38330