def fileno():
        """
        @raise: If the descriptor no longer has a valid file descriptor
            number associated with it.

        @return: The platform-specified representation of a file descriptor
            number.  Or C{-1} if the descriptor no longer has a valid file
            descriptor number associated with it.  As long as the descriptor
            is valid, calls to this method on a particular instance must
            return the same value.
        """

        @raise: If the descriptor no longer has a valid file descriptor
            number associated with it.

        @return: The platform-specified representation of a file descriptor
            number.  Or C{-1} if the descriptor no longer has a valid file
            descriptor number associated with it.  As long as the descriptor
            is valid, calls to this method on a particular instance must
            return the same value.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/interfaces.py