是什么导致了这个回溯？

Question

可能是重复的问题：
在Python/AppEngine中传递GET数据时出现UnicodeDecodeError

我在本地和生产环境中提交表单时遇到了以下错误信息。你能告诉我应该从哪里入手，或者我应该开始添加调试语句，看看代码中异常发生在哪里吗？

 -->  --> -->

Traceback (most recent call last):
  File "/media/Lexar/montao/google/appengine/tools/dev_appserver.py", line 3858, in _HandleRequest
    self._Dispatch(dispatcher, self.rfile, outfile, env_dict)
  File "/media/Lexar/montao/google/appengine/tools/dev_appserver.py", line 3792, in _Dispatch
    base_env_dict=env_dict)
  File "/media/Lexar/montao/google/appengine/tools/dev_appserver.py", line 580, in Dispatch
    base_env_dict=base_env_dict)
  File "/media/Lexar/montao/google/appengine/tools/dev_appserver.py", line 2918, in Dispatch
    self._module_dict)
  File "/media/Lexar/montao/google/appengine/tools/dev_appserver.py", line 2822, in ExecuteCGI
    reset_modules = exec_script(handler_path, cgi_path, hook)
  File "/media/Lexar/montao/google/appengine/tools/dev_appserver.py", line 2704, in ExecuteOrImportScript
    script_module.main()
  File "/media/Lexar/montao/classifiedsmarket/main.py", line 2497, in main
    util.run_wsgi_app(application)
  File "/media/Lexar/montao/google/appengine/ext/webapp/util.py", line 98, in run_wsgi_app
    run_bare_wsgi_app(add_wsgi_middleware(application))
  File "/media/Lexar/montao/google/appengine/ext/webapp/util.py", line 116, in run_bare_wsgi_app
    result = application(env, _start_response)
  File "/media/Lexar/montao/google/appengine/ext/webapp/__init__.py", line 655, in __call__
    response.wsgi_write(start_response)
  File "/media/Lexar/montao/google/appengine/ext/webapp/__init__.py", line 274, in wsgi_write
    body = self.out.getvalue()
  File "/usr/lib/python2.6/StringIO.py", line 270, in getvalue
    self.buf += ''.join(self.buflist)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 3: ordinal not in range(128)

Answer 1

这个错误出现在/usr/lib/python2.6/StringIO.py文件里，也就是Python的StringIO模块。我们不需要深入研究这个文件（第49行）就能找到这个警告：

The StringIO object can accept either Unicode or 8-bit strings, but
mixing the two may take some care. If both are used, 8-bit strings that
cannot be interpreted as 7-bit ASCII (that use the 8th bit) will
cause a UnicodeError to be raised when getvalue() is called.

太好了！这个警告在getvalue()方法中又出现了一次。注意，这个警告是很久以前的；它提到的是UnicodeError，而不是UnicodeDecodeError，但大致意思是一样的。

我建议你修改这个模块，让它在出错时显示出错误发生时的内容。把第270行的有问题的代码包裹起来，像这样：

if self.buflist:
    try:
        self.buf += ''.join(self.buflist)
    except UnicodeDecodeError:
        import sys
        print >> sys.stderr, "*** error context: buf=%r buflist=%r" % (self.buf, self.buflist)
        raise
    self.buflist = []
return self.buf

如果你觉得直接修改Python自带的模块让你感到不安，可以把修改后的版本放在一个比/usr/lib/python2.6更靠前的目录中，这样Python会优先使用你的版本。

这里有一个混合使用非ASCII的str和unicode的例子：

>>> from StringIO import StringIO
>>> f = StringIO()
>>> f.write('ascii')
>>> f.write(u'\u1234'.encode('utf8'))
>>> f.write(u'\u5678')
>>> f.getvalue()
*** error context: buf='' buflist=['ascii', '\xe1\x88\xb4', u'\u5678']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\python26\lib\StringIO.py", line 271, in getvalue
    self.buf += ''.join(self.buflist)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe1 in position 0: ordinal not in range(128)
>>>

然后你可以运行你的应用程序，查看buflist里的内容：哪些是你写入的数据，哪些是GAE提供的。你需要查看GAE的文档，看看它是期待str类型的内容（用什么编码？）还是unicode类型的内容，并相应地调整你的代码。