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proxy_bypass_registry in urllib.py does not handle the ProxyOverride registry value properly: it treats an empty override as *, i.e. bypass the proxy for all hosts. This behavior does not match other programs (e.g. Chrome) and can be easily obtained by specify * for the override. One fix would be to ignore empty tests, for example:
 ....
 for test in proxyOverride:
 if test:
 if test == '<local>':
 ...
 return 0
 ....
Perhaps whitespace should be stripped as well.
The problem arises because fiddler2 leaves a trailing ; on the ProxyOverride string.
One possible workaround is to set
 urllib.proxy_bypass = lambda h: 0
to disable bypass checking. Another alternative would be to specify the proxy settings in the http_proxy environment variable (proxy_bypass_registry is not called in this case).