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abspath() is implemented using nt._getfullpathname() which calls GetFullPathNameA().
> The returned path with question marks is completely useless.
Can you open the file using such filename? If no, I agree that the result is useless.
> It's better that python throw an error than return the question marks.
Python is currently a thin wrapper on the Windows API. Windows doesn't consider that a filename with question marks as an error.
http://msdn.microsoft.com/en-us/library/windows/desktop/aa364963%28v=vs.85%29.aspx
Python can maybe uses GetFullPathNameW() and encode manually the filename using its strict MBCS codec. MBCS codec is strict since Python 3.2: it raises a UnicodeEncodeError if the string cannot be encoded.