在使用Request上传文件的时候碰到如下错误提示：
2013-12-20 20:51:09,235 __main__ ERROR 'ascii' codec can't decode byte 0xe7 in position 27379: ordinal not in range(128)
Traceback (most recent call last):
File "server_merge.py", line 251, in avml_storage
result_f , result_m = avml_storage.uploadData( storage_key, xml_content )
File "/opt/ResultCollector/app/utils/ADFSAvml.py", line 33, in uploadData
r = requests.post(uploadUrl, files=f)
File "/usr/local/lib/python2.7/site-packages/requests-1.2.3-py2.7.egg/requests/api.py", line 88, in post
return request('post', url, data=data, **kwargs)
File "/usr/local/lib/python2.7/site-packages/requests-1.2.3-py2.7.egg/requests/api.py", line 44, in request
return session.request(method=method, url=url, **kwargs)
File "/usr/local/lib/python2.7/site-packages/requests-1.2.3-py2.7.egg/requests/sessions.py", line 335, in request
resp = self.send(prep, **send_kwargs)
File "/usr/local/lib/python2.7/site-packages/requests-1.2.3-py2.7.egg/requests/sessions.py", line 438, in send
r = adapter.send(request, **kwargs)
File "/usr/local/lib/python2.7/site-packages/requests-1.2.3-py2.7.egg/requests/adapters.py", line 292, in send
timeout=timeout
File "/usr/local/lib/python2.7/site-packages/requests-1.2.3-py2.7.egg/requests/packages/urllib3/connectionpool.py", line 428, in urlopen
body=body, headers=headers)
File "/usr/local/lib/python2.7/site-packages/requests-1.2.3-py2.7.egg/requests/packages/urllib3/connectionpool.py", line 280, in _make_request
conn.request(method, url, **httplib_request_kw)
File "/usr/local/lib/python2.7/httplib.py", line 946, in request
self._send_request(method, url, body, headers)
File "/usr/local/lib/python2.7/httplib.py", line 987, in _send_request
self.endheaders(body)
File "/usr/local/lib/python2.7/httplib.py", line 940, in endheaders
self._send_output(message_body)
File "/usr/local/lib/python2.7/httplib.py", line 801, in _send_output
msg += message_body
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe7 in position 27379: ordinal not in range(128)
从traceback很容易辨别出来，发生错误的位置应该是在httplib.py中的_send_output函数中，可能是将unicode 和 str 混用导致的，通过跟踪发现 msg是unicode类型，message_body是str类型。
继续往前追溯，发现Requests将Content-Length设置为unicode，可能是为了方便操作，Request在compat.py中将str做了转义：
if is_py2:
from urllib import quote, unquote, quote_plus, unquote_plus, urlencode
from urlparse import urlparse, urlunparse, urljoin, urlsplit, urldefrag
from urllib2 import parse_http_list
import cookielib
from Cookie import Morsel
from StringIO import StringIO
from .packages.urllib3.packages.ordered_dict import OrderedDict
builtin_str = str
bytes = str
str = unicode
basestring = basestring
numeric_types = (int, long, float)
所以Requests中使用的str都是unicode类型的数据，通过_output、_send_output以及putheader几个函数可以得知问题可能出在putheader函数中对header的处理上，很有可能是没有将unicode转换成str所导致的。
putheader代码如下：
def putheader(self, header, value):
"""Send a request header line to the server.
For example: h.putheader('Accept', 'text/html')
"""
if self.__state != _CS_REQ_STARTED:
raise CannotSendHeader()
str = '%s: %s' % (header, value)
self._output(str)
偶然的机会发现同样的操作方式，使用Request上传文件在另外一台机器上好用，比较两台机器的环境发现只有Python的版本不一致，一个是Python2.7， 一个是Python2.7.3，对比httplib.py发现在功能运行正常的机器（Python2.7.3）上putheader的代码如下：
def putheader(self, header, *values):
"""Send a request header line to the server.
For example: h.putheader('Accept', 'text/html')
"""
if self.__state != _CS_REQ_STARTED:
raise CannotSendHeader()
hdr = '%s: %s' % (header, '\r\n\t'.join([str(v) for v in values]))
self._output(hdr)
通过对比可以很容易看出，Python 2.7.3已经对传入的value做了显式转换，从未避免了某些潜在的错误。
又验证了几个版本，发现 Python 2.6.6、Python2.7是使用Requests库的时候都存在问题， Python 2.7.3、Python2.7.5 都没有问题。建议将Python版本升级至2.7.3以上