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The mac-related code in urllib is correct, it uses the SystemConfiguration framework on MacOSX to get the proxy settings. That said, the code in _scproxy is not entirly correct, it makes the wrong assumption w.r.t. the value of exclude_simple when the corresponding value is not in the settings retrieved from the SystemConfiguration framework.
The tests seem to make assumptions on how the proxy-detecting code functions, although I don't understand what's going on here (that is, I haven't waded through the test code and urllib to trace the code)
The patch "issue-8455-scproxy.patch" ensures that the tests pass, unless the user changed their proxy settings and selected the checkbox "Exclude simple hostnames" (System Preferernces -> Network -> First network adaptor -> Advanced... -> Proxies)
Please test if the patch solves the issue for you as well. 
NOTE: as I mention in the second paragraph the tests seem to make unwarrented assumptions about their environment. Further proof for this:
when you set "http_proxy=localhost:12345" in the environment on a Unixy system the tests will fail as well (although a different subset of the tests).