在Google App Engine获取cookie时引发UnicodeDecodeError

Question

我有一个用Python做的GAE项目，在我的一个请求处理器中，我用以下代码设置了一个cookie：

self.response.headers['Set-Cookie'] = 'app=ABCD; expires=Fri, 31-Dec-2020 23:59:59 GMT'

我在Chrome浏览器中查看，发现cookie确实存在，所以看起来是成功的。

然后在另一个请求处理器中，我想获取这个cookie来检查一下：

appCookie = self.request.cookies['app']

执行这行代码时出现了以下错误：

UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 1962: ordinal not in range(128)

看起来它试图用ASCII编码来解码传入的cookie信息，而不是用UTF-8。

我该如何强制Python使用UTF-8来解码这个呢？

作为一个Python和Google App Engine的新手（但在其他语言上有经验的程序员），还有没有其他与Unicode相关的注意事项我需要了解的？

以下是完整的错误追踪信息：

Traceback (most recent call last):
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/tools/dev_appserver.py", line 4144, in _HandleRequest
    self._Dispatch(dispatcher, self.rfile, outfile, env_dict)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/tools/dev_appserver.py", line 4049, in _Dispatch
    base_env_dict=env_dict)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/tools/dev_appserver.py", line 616, in Dispatch
    base_env_dict=base_env_dict)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/tools/dev_appserver.py", line 3120, in Dispatch
    self._module_dict)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/tools/dev_appserver.py", line 3024, in ExecuteCGI
    reset_modules = exec_script(handler_path, cgi_path, hook)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/tools/dev_appserver.py", line 2887, in ExecuteOrImportScript
    exec module_code in script_module.__dict__
  File "/Users/ken/hgdev/juicekit/main.py", line 402, in <module>
    main()
  File "/Users/ken/hgdev/juicekit/main.py", line 399, in main
    run_wsgi_app(application)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/webapp/util.py", line 98, in run_wsgi_app
    run_bare_wsgi_app(add_wsgi_middleware(application))
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/webapp/util.py", line 116, in run_bare_wsgi_app
    result = application(env, _start_response)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/webapp/__init__.py", line 721, in __call__
    response.wsgi_write(start_response)
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/webapp/__init__.py", line 296, in wsgi_write
    body = self.out.getvalue()
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/StringIO.py", line 270, in getvalue
    self.buf += ''.join(self.buflist)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 1962: ordinal not in range(128)

Answer 1

首先，设置在 cookies 中的任何 unicode 值时，要进行编码。你还需要加上引号，以防这些值破坏了头部信息：

import urllib

# This is the value we want to set.
initial_value = u'äëïöü'
# WebOb version that comes with SDK doesn't quote cookie values
# in the Response, neither webapp.Response. So we have to do it.
quoted_value = urllib.quote(initial_value.encode('utf-8'))

rsp = webapp.Response()
rsp.headers['Set-Cookie'] = 'app=%s; Path=/' % quoted_value

接下来，我们来读取这个值。为了测试它，创建一个假的 Request 来测试我们设置的 cookie。这个代码是从一个真实的单元测试中提取出来的：

cookie = rsp.headers.get('Set-Cookie')
req = webapp.Request.blank('/', headers=[('Cookie', cookie)])

# The stored value is the same quoted value from before.
# Notice that here we use .str_cookies, not .cookies.
stored_value = req.str_cookies.get('app')
self.assertEqual(stored_value, quoted_value)

我们的值仍然是编码过的并且加了引号。我们需要把它们反过来处理，才能得到最初的值：

# And we can get the initial value unquoting and decoding.
final_value = urllib.unquote(stored_value).decode('utf-8')
self.assertEqual(final_value, initial_value)

如果可以的话，建议使用 webapp2。webob.Response 可以帮你处理所有关于引号和设置 cookies 的繁琐工作，而且你可以直接设置 unicode 值。有关这些问题的总结，可以在 这里 查看。

Answer 2

你想用 decode 函数大概是这样的（感谢 @agf 的分享）：

self.request.cookies['app'].decode('utf-8')

来自官方的 Python 文档（还有一些补充的细节）：

Python 的 8 位字符串有一个 .decode([编码], [错误处理]) 方法，这个方法可以用你指定的编码来解释这个字符串。下面的例子展示了字符串是如何转换成 Unicode，然后再转换回 8 位字符串的：

>>> u = unichr(40960) + u'abcd' + unichr(1972)   # Assemble a string
>>> type(u), u                                   # Examine
(<type 'unicode'>, u'\ua000abcd\u07b4')
>>> utf8_version = u.encode('utf-8')             # Encode as UTF-8
>>> type(utf8_version), utf8_version             # Examine
(<type 'str'>, '\xea\x80\x80abcd\xde\xb4')
>>> u2 = utf8_version.decode('utf-8')            # Decode using UTF-8
>>> u == u2                                      # The two strings match
True