How to send JSON as part of multipart POST-request

Question

I have following POST-request form (simplified):

POST /target_page HTTP/1.1  
Host: server_IP:8080
Content-Type: multipart/form-data; boundary=AaaBbbCcc

--AaaBbbCcc
Content-Disposition: form-data; name="json" 
Content-Type: application/json

{ "param_1": "value_1", "param_2": "value_2"}

--AaaBbbCcc
Content-Disposition: form-data; name="file"; filename="..." 
Content-Type: application/octet-stream

<..file data..>
--AaaBbbCcc--

I try to send POST-request with requests:

import requests
import json

file = "C:\\Path\\To\\File\\file.zip"
url = 'http://server_IP:8080/target_page'


def send_request():
    headers = {'Content-type': 'multipart/form-data; boundary=AaaBbbCcc'}

    payload = { "param_1": "value_1", "param_2": "value_2"}

    r = requests.post(url, files={'json': (None, json.dumps(payload), 'application/json'), 'file': (open(file, 'rb'), 'application/octet-stream')}, headers=headers)

    print(r.content)

if __name__ == '__main__':
    send_request()

but it returns status 400 with following comment:

Required request part \'json\' is not present.
The request sent by the client was syntactically incorrect.

Please point on my mistake. What should I change to make it work?

Solution

You are setting the header yourself, including a boundary. Don't do this; requests generates a boundary for you and sets it in the header, but if you already set the header then the resulting payload and the header will not match. Just drop you headers altogether:

def send_request():
    payload = {"param_1": "value_1", "param_2": "value_2"}
    files = {
         'json': (None, json.dumps(payload), 'application/json'),
         'file': (os.path.basename(file), open(file, 'rb'), 'application/octet-stream')
    }

    r = requests.post(url, files=files)
    print(r.content)

Note that I also gave the file part a filename (the base name of the file path`).

For more information on multi-part POST requests, see the advanced section of the documentation.

Comments

[jsbueno]

If anything the fact the O.P. is passing the boundary pattern inside an opaque string, and still expecting the library to reuse that pattern, should be a goog clue that something is wrong.

[Khamidulla]

Using this code snippet i was able to send request. However at django backend says AttributeError: 'WSGIRequest' object has no attribute 'data'. Do you have any idea why?

[Martijn Pieters]

@Khamidulla: that's not a problem with this code. Something on the Django server has an error, there is no data attribute on the Django Request object.

[Khamidulla]

@MartijnPieters I will publish my question and send you link later today. Thank you for you response.

[Scaramouche]

hello, is the data supposed to be received via body in the server? it's arriving empty, thank you

[Martijn Pieters]

@Scaramouche: yes, POST data (however it is encoded) is sent in the request body. Replace your URL with https://httpbin.org/post to receive a JSON response that echoes what you sent to verify that you are sending a valid request; this could be a server-side issue rather than a Requests issue.

[JavaSa]

@MartijnPieters; how this request would look if I have multiple files with one metadata json?

[Martijn Pieters]

@JavaSa: Use a tuple with (partname, (filename, filedata, content_type)) elements, that way part names don't need to be unique . Do look at the documentation I linked to from my answer, which includes that format.