Post request with multiple parameters using Twisted Web Client

I would like to send a POST request with multiple parameters using Twisted Web Client :

image : image
metadata : json document with meta data I need to use pure Twisted without external libraries like Treq and requests.

At the moment, I can send only one parameter and tried few ways without success.

Do someone know how to change body to achieve this goal ?

from __future__ import print_function

from twisted.internet import reactor
from twisted.web.client import Agent
from twisted.web.http_headers import Headers

from bytesprod import BytesProducer

agent = Agent(reactor)
body = BytesProducer(b"hello, world")
d = agent.request(
    b'POST',
    b'http://httpbin.org/post',
    Headers({'User-Agent': ['Twisted Web Client Example'],
             'Content-Type': ['text/x-greeting']}),
    body)

def cbResponse(ignored):
    print('Response received')
d.addCallback(cbResponse)

def cbShutdown(ignored):
    reactor.stop()
d.addBoth(cbShutdown)

reactor.run()

Solution

You need to specify how you would like the parameters encoded. If you want to to submit them like a browser form, you need to application/x-www-form-urlencoded or multipart/form-data encode the data. The former is generally for short data - and since of your parameters is "image" it probably isn't short. So you should multipart/form-data the data.

Once you have, you just declare this in the request head and include the encoded data in the body.

For example,

body = multipart_form_encoded_body_producer(your_form_fields))
d = agent.request(
    b'POST',
    b'http://httpbin.org/post',
    Headers({'User-Agent': ['Twisted Web Client Example'],
             'Content-Type': ['multipart/form-data']}),
    body)

Conveniently, treq provides a multipart/form-data encoder

So multipart_form_encoded_body_producer(...) probably looks something like:

MultiPartProducer([
    ("image", image_data),
    ("metadata", some_metadata),
    ...
])

You mentioned that you can't use Treq. You didn't mention why. I recommend using Treq or at least finding another library that can do the encoding for you. If you can't do that for some unreasonable reason, you'll have to implement multipart/form-data encoding yourself. It is reasonably well documented and of course there are multiple implementations you can also use as references and interoperability testing tools.