The no_proxy environment variable works in python as a case
sensitive suffix check.

Curl handles this variable as a case insensitive hostname check.

Case sensitivity appears to be in conflict with the DNS Case
Insensitivity RFC (https://tools.ietf.org/html/rfc4343).

While the suffix check is documented
(https://docs.python.org/3/library/urllib.request.html), this
seems to be problematic and inconsistent with other tools on the
system.

I believe the ideal solution would be to have proxy_bypass be a
method of ProxyHandler so that it can be overridden without
dependence on undocumented methods. This would also allow
for the requested behavior to be added without breaking backwards
compatibility.

No, urllib.request does not handle the no_proxy environment in case-sensitive way. It first checks if any environment_variable.lower() ends with "_proxy", and then checks if there is any environment_variable ends with "_proxy". So it works as a case insensitive check, but the lowercase one will override others, e.g, no_proxy overrides no_PROXY. But the document is misleading, it should be clarified.

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.

Ohh, it's my fault to misunderstand. I think your thinking is reasonable. Curl handles the suffix check insensitively.

I write a patch to fix this.

It seems on Windows and MacOS the behaviour is right. On MacOS it leaves the matching to fnmatch. On Windows it uses case insensitive regular matching.

The code has changed recently. I update the patch to reveal the change.

I think this patch looks okay. It fixes the case insensitivity problem, but there is also the string suffix problem (see the myexample.com demo). I think it is reasonable for example.com to match my.example.com, but not myexample.com.

Yes, you are right. I don't notice that. I will try to fix it.

I update the patch to use regular expression to handle both case and suffix. This regular expression has the function of curl's check_noproxy. And I think a separate test case is better here.

Update to improve the test case.

Thanks for your comment martin. Using re.escape is really a good advise.

This version looks okay to me, thanks.

New changeset ca882ee68d46 by Martin Panter in branch '3.5':
Issue #26864: Fix case insensitivity and suffix comparison with no_proxy
https://hg.python.org/cpython/rev/ca882ee68d46

New changeset 1ceb91974dc4 by Martin Panter in branch 'default':
Issue #26864: Merge no_proxy fixes from 3.5
https://hg.python.org/cpython/rev/1ceb91974dc4

New changeset a1aad42f1195 by Martin Panter in branch '2.7':
Issue #26864: Fix case insensitivity and suffix comparison with no_proxy
https://hg.python.org/cpython/rev/a1aad42f1195