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This is related to issue 15826.
When run with the -b option, some glob.py and os.py functions give warnings due to byte-to-string conversions:
amoura@amoura-laptop:~/cpython$ ./python -b -c "import glob; glob.glob(b'cover*/glob.cover')"
/home/amoura/cpython/Lib/glob.py:64: BytesWarning: Comparison between bytes and string
 if basename == '':
amoura@amoura-laptop:~/cpython$ ./python -b -c "import os; os.makedirs(b'tst/making/dirs')"
/home/amoura/cpython/Lib/os.py:266: BytesWarning: Comparison between bytes and string
 if tail == cdir: # xxx/newdir/. exists if xxx/newdir exists
The attached patch fixes this.
There is a rather more mysterious phenomenon with exceptions (which is triggered by test_exceptions for ImportException, but it happens for any Exception class):
>>> e = Exception(b'aaa')
[60596 refs]
>>> e.args[0]
b'aaa'
[60601 refs]
>>> str(e)
__main__:1: BytesWarning: str() on a bytes instance
"b'aaa'"
[60615 refs]
>>> e.args[0]
b'aaa'
[60615 refs]
>>> str(e)
"b'aaa'"
[60615 refs]
>>> e.args[0]
b'aaa'
[60615 refs]
In other words, if a bytes argument is given to the exception, the first call to str triggers the warning, but further calls don't. Is this worth pursuing?

Not repeating warnings from the same place is the default warning behavior. You can get all of them by passing -Wall.

- if basename == '':
+ if len(basename) == 0:
This should be "if not basename:"
- if tail == curdir:
+ cdir = curdir
+ if isinstance(tail, bytes):
+ cdir = bytes(curdir, 'ASCII')
+ if tail == cdir:
This will raise an error if curdir is a non-ascii str, so, unless the same error was already raised later in the code, this is backward incompatible.

The first bug fixed in issue16696.

> This will raise an error if curdir is a non-ascii str, so, unless the same error was already raised later in the code, this is backward incompatible.
On all supported platforms curdir is a ascii str (':' on Mac Classic, '.' on all other). The same idiom used in glob module.

New changeset 9458a516f769 by Serhiy Storchaka in branch '3.2':
Issue #15845: Fix comparison between bytes and string.
http://hg.python.org/cpython/rev/9458a516f769
New changeset f6cf2985348a by Serhiy Storchaka in branch '3.3':
Issue #15845: Fix comparison between bytes and string.
http://hg.python.org/cpython/rev/f6cf2985348a
New changeset 51e60d9ee389 by Serhiy Storchaka in branch 'default':
Issue #15845: Fix comparison between bytes and string.
http://hg.python.org/cpython/rev/51e60d9ee389 

Fixed. Thank your for report and patch, Alessandro.