>>> url = "http://httpbin.org/post"
>>> r = requests.post(url, data="myscreename:mypassword")
>>> print r.text
{
  "args": {}, 
  "data": "myscreename:mypassword", 
  "files": {}, 
  "form": {}, 
  "headers": {
    "Accept": "*/*", 
    "Accept-Encoding": "gzip, deflate", 
    "Content-Length": "22", 
    "Host": "httpbin.org", 
    "User-Agent": "python-requests/2.5.1 CPython/2.7.6 Darwin/14.3.0"
  }, 
  "json": null, 
  "origin": "16.7.5.3", 
  "url": "http://httpbin.org/post"
}

>>> r = requests.post(url, auth=("myscreename", "mypassword"))
>>> print r.text
{
  "args": {}, 
  "data": "", 
  "files": {}, 
  "form": {}, 
  "headers": {
    "Accept": "*/*", 
    "Accept-Encoding": "gzip, deflate", 
    "Authorization": "Basic bXlzY3JlZW5hbWU6bXlwYXNzd29yZA==", 
    "Content-Length": "0", 
    "Host": "httpbin.org", 
    "User-Agent": "python-requests/2.5.1 CPython/2.7.6 Darwin/14.3.0"
  }, 
  "json": null, 
  "origin": "16.7.5.3", 
  "url": "http://httpbin.org/post"
}