Position the circle tangent to the parent circle with the line connecting the centers of the two circles meeting the x axis at angle theta.

def calcPosition(self,parent_circle):
        ''' Position the circle tangent to the parent circle with the line connecting the centers of the two circles meeting the x axis at angle theta. '''
        if r not in self:
            raise AttributeError("radius must be calculated before position.")
        if theta not in self:
            raise AttributeError("theta must be set before position can be calculated.")
        x_offset = math.cos(t_radians) * (parent_circle.r + self.r)
        y_offset = math.sin(t_radians) * (parent_circle.r + self.r)
        self.x = parent_circle.x + x_offset
        self.y = parent_circle.y + y_offset