determine the angle of this point on a circle, measured in radians (presume values represent a Vector)

def angle2d(self):
        """determine the angle of this point on a circle, measured in radians (presume values represent a Vector)"""
        if self.x==0:
            if   self.y<0:  return       math.pi/2.0*3
            elif self.y>0:  return       math.pi/2.0
            else:           return 0
        elif self.y==0:
            if   self.x<0:  return       math.pi
           #elif self.x>0:  return 0
            else:           return 0
        ans = math.atan( self.y / self.x )
        if self.x > 0:
            if self.y>0:    return ans
            else:           return ans + math.pi*2.0
        else:               return ans + math.pi