Using POST and urllib2 to access web API

Question

I am trying to access a web API using a POST technique. I AM able to access it using a GET technique, but the API owners tell me that certain functionality only works with POST. Unfortunately I can't seem to get POST working.

Here's what works with GET:

API_URL = "http://example.com/api/"

def call_api(method, **kwargs):
    url = API_URL + method
    if kwargs:
        url += '?' + urllib.urlencode(kwargs)
    req = urllib2.Request(url)
    auth = 'Basic ' + base64.urlsafe_b64encode("%s:%s" % (USER, PASS))
    req.add_header('Authorization', auth)
    return urllib2.urlopen(req)

Here's what does NOT work with POST (causes HTTP 400 error):

API_URL = "http://example.com/api/"

def call_api(method, **kwargs):
    url = API_URL + method
    data=''
    if kwargs:
        data=urllib.urlencode(kwargs)
    req = urllib2.Request(url, data)
    auth = 'Basic ' + base64.urlsafe_b64encode("%s:%s" % (USER, PASS))
    req.add_header('Authorization', auth)
    return urllib2.urlopen(req)

Does anything jump out at anyone as being inherently incorrect in the POST code? I've never done a POST call before but everything I've read seems to suggest that my code is reasonable. Is there some different way I'm supposed to do the add_header thing for the authorization if I'm using POST?

Answer 1

With urllib2 you need to add the data to the POST body:

def call_api(method, **kwargs):
    url = API_URL + method
    req = urllib2.Request(url)

    if kwargs:
        req.add_data(urllib.urlencode(kwargs))

    auth = 'Basic ' + base64.urlsafe_b64encode("%s:%s" % (USER, PASS))
    req.add_header('Authorization', auth)

    # req.get_method() -> 'POST'

    return urllib2.urlopen(req)

Answer 2

As @sneeu notes above, it's the act of adding the data to be posted to the request that converts the request from a GET into a POST.

However, this still assumes that what the API is expecting to receive in the POST body is form-encoded data. Many more recent APIs that I've worked with are expecting something else in there (XML or JSON, most commonly).

Can you verify what that API is expecting to receive as a data payload?