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Following the documentation,
os.path.normcase(path)
Normalize the case of a pathname. On Unix and Mac OS X, this returns the path unchanged; on case-insensitive filesystems, it converts the path to lowercase. On Windows, it also converts forward slashes to backward slashes.
on Mac OS X,
>>> import os
>>> os.name
'posix'
Checking through, Lib/posixpath.py,
# Normalize the case of a pathname. Trivial in Posix, string.lower on Mac.
# On MS-DOS this may also turn slashes into backslashes; however, other
# normalizations (such as optimizing '../' away) are not allowed 
# (another function should be defined to do that).
def normcase(s):
 """Normalize case of pathname. Has no effect under Posix"""
 # TODO: on Mac OS X, this should really return s.lower().
 return s
Why on Mac OS X, it should return s.lower() ?
Also to raise Error for None case we can probably change normcase() in Lib/posixpath.py as,
def normcase(s):
 """Normalize case of pathname. Has no effect under Posix"""
 if s is None: 
 raise AttributeError
 return s