def __init__(self, speed):
        """
        Initializes a L{Speed} object.

        @param speed: The speed that this object represents, expressed in
            meters per second.
        @type speed: C{float}

        @raises ValueError: Raised if C{speed} is negative.
        """
        if speed < 0:
            raise ValueError("negative speed: %r" % (speed,))

        _BaseSpeed.__init__(self, speed)

        Initializes a L{Speed} object.

        @param speed: The speed that this object represents, expressed in
            meters per second.
        @type speed: C{float}

        @raises ValueError: Raised if C{speed} is negative.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/positioning/base.py