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On OS X, urllib.request in Python 3 is supposed to use the operating 
system's proxy configuration by default, unless overridden by 
environment variables or by the caller providing an explicit proxy 
configuration.
In Python 2, urllib (and, indirectly, urllib2) were modified to use the 
OS X SystemConfiguration framework API instead of the obsolete classic 
MacOS Internet Configuration via the deprecated Macintosh ic library 
module, which was removed in Python 3.
However, in the transition from Python 2 urllib/urllib2 to Python 3 
urllib.request, the changes to the darwin-specific getproxies functions 
were not made. So, at the moment, Python 3 urllib.request tries to 
import the non-existant ic module and silently fails, effectively 
ignoring the system proxy configuration. The solution is to forward 
port the corresponding code from Python 2 urllib along with the helper 
module _scproxy.c.

I agree that _scproxy should be ported to the 3.x trees. Such a port 
should be fairly straightforward, but is harder than just copying files 
over due to the str/unicode changes in the 3.x tree.

I've committed a port of _scproxies and the related code in urllib in r80198 (3.2), r80199 (3.1)
BTW. With some luck this should already include a fix for the crash your seeing on OSX 10.3 with the trunk and 2.6.