Return multipart/formdata containing png file and markdown as a flask response

Question

Hi I am trying to return multipart from data from a get request. However I am unable to return a MultipartEncoder object as a response. I have attempted the solution at Can I serve a multipart http response in Flask?

The below code only returns the 'toPersonEmail' field. How does one return m as a response in Flask?

@app.route("/multipart", methods=['GET'])
def send_multipart():
     m = MultipartEncoder(fields={ 'markdown': "> Block quotes are written like so in markdown.",
                                   'toPersonEmail':'d@d.com',
                                   'files': ("pnggrad16rgb.png", open("pnggrad16rgb.png", 'rb'),
                                   'image.png')})

     
     return Response(m.fields, mimetype=m.content_type)

Answer 1

Flask has no specific provision for multipart mime responses; multipart/form is a mimetype normally only used in a POST request to the server, not in a response from a HTTP server.

If you must return valid multipart mime data, render the body to bytes and set the right headers on a response object; the `(

m = MultipartEncoder(fields={ 'markdown': "> Block quotes are written like so in markdown.",
                               'toPersonEmail':'d@d.com',
                               'files': ("pnggrad16rgb.png", open("pnggrad16rgb.png", 'rb'),
                               'image.png')})

return (m.to_string(), {'Content-Type': m.content_type})

If you wanted to stream the response, you'd have to supply your own generator function that reads from the multipart encoder in chunks:

def chunked_reader(f, chunksize=2 ** 20):  # 1Mb chunks
    while True:
        chunk = f.read(chunksize)
        if not chunk:
            return
        yield chunk

and use that to wrap your MultipartEncoder() instance when streaming response data:

# include the Content-Length header, even though we are streaming
return Response(
    chunked_reader(m), content_type=m.content_type,
    headers={'Content-Length': m.len})

The MultipartEncoder() object supports a .read() method just like files do.