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3.0c3 Manual (as with 2.x):
os.extsep 
The character which separates the base filename from the extension; for
example, the '.' in os.py. Also available via os.path.
3.0c3
>>> import os
>>> os.extsep
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'extsep'
>>> os.path.extsep
'.'
os.extsep apparently was present in 2.x
Mention of os.extsep was also present in 2.x docstring and removed in
3.0 docstring, so perhaps this was intentional, but it does not make
much sense, so I am provisionally marking this as a library bug.