multipart/form-data类型的POST实体结构相对来说（常规的POST正文采用application/x-www-form-urlencoded格式）比较复杂，它常用于文件上传。

 下面是一个multipart/form-data格式的POST实体示例：

-----------------------------114782935826962
Content-Disposition: form-data; name="dv_deputy"

e5471d92ebec3654ee6b90d131d296f4
-----------------------------114782935826962
Content-Disposition: form-data; name="form_id"

submit_new_resume
-----------------------------114782935826962
Content-Disposition: form-data; name="resumator-subdomain-value"

app
-----------------------------114782935826962
Content-Disposition: form-data; name="resumator-tags-value"


-----------------------------114782935826962
Content-Disposition: form-data; name="resumator-job-value"

0
-----------------------------114782935826962
Content-Disposition: form-data; name="manual"

true

-----------------------------114782935826962
Content-Disposition: form-data; name="resumator-eeo_disability-value"

0
-----------------------------114782935826962--
最后这里还有一个空行

封装代码如下：

# coding: utf-8
# util.py

import os
import mimetypes
import mimetools

def get_content_type(filepath):
    return mimetypes.guess_type(filepath)[0] or 'application/octet-stream'

def encode_multipart_formdata(fields, files=[]):
    """
    fields is a sequence of (name, value) elements for regular form fields.
    files is a sequence of (name, filepath) elements for data to be uploaded as files
    Return (content_type, body) ready for httplib.HTTP instance
    """
    BOUNDARY = mimetools.choose_boundary()
    CRLF = '\r\n'
    L = []
    for (key, value) in fields:
        L.append('--' + BOUNDARY)
        L.append('Content-Disposition: form-data; name="%s"' % key)
        L.append('')
        L.append(value)
    for (key, filepath) in files:
        L.append('--' + BOUNDARY)
        L.append('Content-Disposition: form-data; name="%s"; filename="%s"' % (key, os.path.basename(filepath)))
        L.append('Content-Type: %s' % get_content_type(filepath))
        L.append('')
        L.append(open(filepath, 'rb').read())
    L.append('--' + BOUNDARY + '--')
    L.append('')
    body = CRLF.join(L)
    content_type = 'multipart/form-data; boundary=%s' % BOUNDARY
    return content_type, body

encode_multipart_formdata返回的content_type应设置为content-type头。body即为封装后的数据。

参考：http://code.activestate.com/recipes/146306/