import requests
url = "http://www.xxxx.net/login"

#参数拼凑,附件上传格式如picurl参数，其他表单参数值拼成tuple格式：
2-tuples (filename, fileobj), 
3-tuples (filename, fileobj, contentype)，
4-tuples (filename, fileobj, contentype, custom_headers)

files = {"username": (None, "billy"), "password": (None, "abcd1234"),
  'picUrl': ('pic.png', open('E:\\download\\pic.png', 'rb'), 'image/png')}

#如需headers，不需要赋值Content-Type,不然可能会报错
res = requests.post(url, files=files)
print res.request.body
print res.request.headers

from requests_toolbelt import MultipartEncoder
import requests

m = MultipartEncoder(
 fields={'field0': 'value', 'field1': 'value',
   'field2': ('filename', open('file.py', 'rb'), 'text/plain')}，
 )
r = requests.post('http://httpbin.org/post', data=m,
     headers={'Content-Type': m.content_type})

from requests_toolbelt import MultipartEncoder
import requests
m = MultipartEncoder(fields={'field0': 'value', 'field1': 'value'})
r = requests.post('http://httpbin.org/post', data=m,
     headers={'Content-Type': m.content_type})