'Set turtle-mode to \'standard\', \'world\' or \'logo\'.'

def _setmode(self, mode=None):

    if (mode is None):
        return self._mode
    if (mode not in ['standard', 'logo', 'world']):
        return
    self._mode = mode
    if (mode in ['standard', 'world']):
        self._angleOffset = 0
        self._angleOrient = 1
    else:
        self._angleOffset = (self._fullcircle / 4.0)
        self._angleOrient = (-1)
