self.assertEqual(self.x.subs(self.y, 3), self.x)
self.assertEqual(self.pi.subs(self.x, 3), self.pi)
self.assertEqual(self.expr.subs(self.x, -3), -2 * self.y)
self.assertEqual(self.x.subs(self.y, 3), self.x)
self.assertEqual(self.pi.subs(self.x, 3), self.pi)
self.assertEqual(self.expr.subs(self.x, -3), -2 * self.y)