I have a Tomcat+Java based server exposing REST APIs. I am writing a client in python to consume those APIs. Everything is fine until I send empty body in POST request. It is a valid use case for us. If I send empty body I get 400 bad request error - Invalid character found in method name [{}POST]. HTTP method names must be tokens. If I send empty request from POSTMAN or Java or CURL it works fine, problem is only when I used python as a client. 

If I use python based server (attached a simple server program for same) then I get 405 Method not allowed instead of 400 that I get using java based server.

Server code is Following is python snippet -

json_object={}
header = {'alias': 'A', 'Content-Type' : 'application/json', 'Content-Length' : '0'} 
resp = requests.post(url, auth=(username, password), headers=header, json=json_object) 
I tried using data as well instead of json param to send payload with not much of success.

I captured the wireshark dumps to undertand it further and found that, the request tomcat received is not as per RFC2616 (https://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html). Especially the part - Request-Line = Method SP Request-URI SP HTTP-Version CRLF Because I could see in from wireshark dumps it looked like - {}POST MY-APP-URI HTTP/1.1

As we can see the empty body is getting prefixed with http-method, hence tomcat reports that as an error. I then looked at python http library code -client.py. Following are relevant details -

File - client.py

Method - _send_output (starting at line # 1001) - It first sends the header at line #1010 and then the body somewhere down in the code. I thought(I could be wrong here) perhaps in this case header is way longer 310 bytes than body 2 bytes, so by the time complete header is sent on wire body is pushed and hence TCP frames are order in such a way that body appears first. To corroborate this I added a delay of 1 second just after sending header line#1011 and bingo, the error disappeared and it started working fine. Not sure if this is completely correct analysis, but can someone in the know can confirm or let me know how to fix this.