def reserve():
        """
        Attempt to open the reserved file descriptor; if this fails
        because of C{EMFILE}, internal state is reset so that another
        reservation attempt can be made.

        @raises: Any exception except an L{OSError} or L{IOError}
            whose errno is L{EMFILE}.
        """

        Attempt to open the reserved file descriptor; if this fails
        because of C{EMFILE}, internal state is reset so that another
        reservation attempt can be made.

        @raises: Any exception except an L{OSError} or L{IOError}
            whose errno is L{EMFILE}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/internet/tcp.py