importlib ignores the PYTHONCASEOK environment variable on Windows. _relax_case checks for b'PYTHONCASEOK' in os.environ, which is never true because os.environ is Unicode. On Windows, _make_relax_case should return a function that instead checks for 'PYTHONCASEOK'.

The following test demonstrates that case-insensitive imports otherwise appear to work correctly in 3.5:

    >>> import sys, importlib, glob
    >>> glob.glob('*.py')
    ['Test.py']

    >>> importlib._bootstrap_external._relax_case()
    False
    >>> import test
    >>> test.__file__
    'C:\\Program Files\\Python35\\lib\\test\\__init__.py'

patched:

    >>> src = "_relax_case = lambda: 'PYTHONCASEOK' in _os.environ"
    >>> exec(src, vars(importlib._bootstrap_external))
    >>> importlib._bootstrap_external._relax_case()
    True
    >>> del sys.modules['test']
    >>> import test
    this is a test
    >>> test.__file__
    'C:\\Temp\\test.py'

It would be better if __file__ were the actual filename "Test.py" instead of "test.py".

Did you forgot to upload a patch?

In python 3.6 from hg it has been fixed.

>>> glob.glob('*.py')
['setup.py', 'test.py']
>>> import test
>>> test.__file__
'C:\\Users\\brian\\Desktop\\cpython\\test.py'
>>>

New changeset bedcb9ec3f26 by Brett Cannon in branch '3.5':
Issue #27083: Respect the PYTHONCASEOK environment variable under
https://hg.python.org/cpython/rev/bedcb9ec3f26

New changeset 777da58794ca by Brett Cannon in branch 'default':
Merge for #27083
https://hg.python.org/cpython/rev/777da58794ca

I fixed this by simply checking for both b'PYTHONCASEOK' and 'PYTHONCASEOK'. Thanks for the bug report, Eryk!

Comparing bytes with str can emit a warning or raise an exception. Checking for b'PYTHONCASEOK' in os.environ looks as a bug to me.

There's no direct comparison to trigger a warning; it's just seeing if a key in a dict exists.

And b'PYTHONCASEOK' is not a bug as that's required for OS X (remember this is technically posix.environ, not os.environ).

Seeing if a key in a dict exists involves the key comparison if there is a key on the same position in the hash table.

I think the best we can do is be platform-specific in what is checked for in the environ dict (e.g. b'PYTHONCASEOK' on darwin, 'PYTHONCASEOK' on win, and I don't know about cygwin), but otherwise one would have to iterate through the keys of the dict and do a type check before doing the comparison to fully avoid any potential warning being triggered.

Does the platform-specific decision of what to look for work for you, Serhiy?

Yes, I think it will work.

This appears to have broken the tests on Windows:

http://buildbot.python.org/all/builders/x86%20Windows7%203.5/builds/1030/steps/test/logs/stdio
======================================================================
FAIL: test_insensitive (test.test_importlib.source.test_case_sensitivity.Frozen_CaseSensitivityTestPEP302)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\cygwin\home\db3l\buildarea\3.5.bolen-windows7\build\lib\test\test_importlib\source\test_case_sensitivity.py", line 65, in test_insensitive
    self.assertIsNotNone(insensitive)
AssertionError: unexpectedly None

(Plus three more failures)

New changeset 6b46c1510bfa by Brett Cannon in branch '3.5':
Fix regressions introduced by fixes for issue #27083.
https://hg.python.org/cpython/rev/6b46c1510bfa

New changeset f4c91b883772 by Brett Cannon in branch 'default':
Merge for #27083
https://hg.python.org/cpython/rev/f4c91b883772

Buildbots are green again and I addressed the key type issue. Thanks for letting me know about the failure, Martin.