Daniel Morrison added the comment: I believe there was a misunderstanding. While the environment variable name is handled in a case insensitive way, the contents of the environment variable is not. Please see some examples below: >>> os.environ['no_proxy'] = 'example.com' >>> urllib.request.proxy_bypass('EXAMPLE.com') 0 >>> urllib.request.proxy_bypass('example.com') 1 Also to clarify the meaning of suffix check: >>> os.environ['no_proxy'] = 'example.com' >>> urllib.request.proxy_bypass('myexample.com') 1 My apologies for my lack of clarity. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue26864> _______________________________________