输入子类并且重写area，按公式计算面积

国庆考核.py

@@ -1,3 +1,15 @@
 class Shape:
     def area(self):
         return 0
+
+class Circle(Shape):
+    def __init__(self, r):
+        self.r = r
+    def area(self):
+        return 3.14 * self.r * self.r
+
+class Square(Shape):
+    def __init__(self, side):
+        self.side = side
+    def area(self):
+        return self.side * self.side