'Set the alpha value used for blending - not supported on all backends.
If ``alpha=None`` (the default), the alpha components of the
foreground and fill colors will be used to set their respective
transparencies (where applicable); otherwise, ``alpha`` will override
them.'

def set_alpha(self, alpha):

    if (alpha is not None):
        self._alpha = alpha
        self._forced_alpha = True
    else:
        self._alpha = 1.0
        self._forced_alpha = False
    self.set_foreground(self._rgb, isRGBA=True)
