No fuss Authorize.Net Python Class, supports auth_only, prior_auth_capture, credit, and void. Please improve at will. Minimally tested, no errors so far. Something nice to add is split tender support, and recurring subscriptions.

authorizenet.py

import urllib2, urllib


class AuthorizeNet():

  def __init__(self, **kwargs):

    self.url = 'https://test.authorize.net/gateway/transact.dll'

    self.api = {
        'x_login':kwargs['login'],
        'x_tran_key':kwargs['tran_key'],
        'x_method':'CC', 
        'x_delim_data':'TRUE',
        'x_duplicate_window':'10',
        'x_delim_char':'|',
        'x_relay_response':'FALSE', 
        'x_version':'3.1'
    }

  def run(self, data):

    data.update(self.api)

    request = urllib2.Request(self.url, urllib.urlencode(data))

    response = urllib2.urlopen(request).read()
    
    return response.split('|')

  def auth(self, **kwargs):

    auth_data = {}

    auth_data['x_type'] = 'AUTH_ONLY'
    auth_data['x_amount'] = unicode(kwargs['amount'])
    auth_data['x_card_num'] = kwargs['card_num']
    auth_data['x_exp_date'] = kwargs['exp_date']
    auth_data['x_card_code'] = kwargs['card_code']
    auth_data['x_zip'] = kwargs['zip']

    return self.run(auth_data)

  def capture(self, **kwargs):

    capture_data = {}

    capture_data['x_type'] = 'PRIOR_AUTH_CAPTURE'
    capture_data['x_trans_id'] = kwargs['trans_id']

    return self.run(capture_data)

  def refund(self, **kwargs):
    
    refund_data = {}

    refund_data['x_type'] = 'CREDIT'
    refund_data['x_trans_id'] = kwargs['x_trans_id']
    refund_data['x_card_num'] = kwargs['x_card_num']

    return self.run(refund_data)

  def void(self, **kwargs):
    
    refund_data = {}

    refund_data['x_type'] = 'VOID'
    refund_data['x_trans_id'] = kwargs['x_trans_id']

    return self.run(refund_data)


#USAGE
#charge = AuthorizeNet(login='XXX', tran_key='XXX')
#print charge.auth(amount=12000, card_num='370000000000002', exp_date='0414', card_code='045', zip='10001')