'Set the alpha tranparencies of the collection.  *alpha* must be
a float or *None*.
ACCEPTS: float or None'

def set_alpha(self, alpha):

    if (alpha is not None):
        try:
            float(alpha)
        except TypeError:
            raise TypeError(u'alpha    must    be    a    float    or    None')
    artist.Artist.set_alpha(self, alpha)
    self._set_facecolor(self._original_facecolor)
    self._set_edgecolor(self._original_edgecolor)
