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 changesetbedcb9ec3f26 by Brett Cannon in branch '3.5':Issue#27083: Respect the PYTHONCASEOK environment variable underhttps://hg.python.org/cpython/rev/bedcb9ec3f26New changeset777da58794ca by Brett Cannon in branch 'default':Merge for#27083https://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 changeset6b46c1510bfa by Brett Cannon in branch '3.5':Fix regressions introduced by fixes for issue#27083.https://hg.python.org/cpython/rev/6b46c1510bfaNew changesetf4c91b883772 by Brett Cannon in branch 'default':Merge for#27083https://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.