python类的继承及重写父类方法

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1.类的继承，我们先来写两个类，他们之间为包含关系。类A为（Tree）树,类B为（Apple）苹果树,类C为（Pear）梨子树

class Tree:
	def __init__(self,name,age):
		self.name = name
		self.age = age

	def grow(self):
		print("{} tree is growing...now it's {} years old".format(self.name,self.age))


	def seed(self):
		print("{} tree is seeding...".format(self.name))


class Apple(Tree):
	"""docstring for ClassName"""
	def __init__(self, name, age, color):
		super().__init__(name,age)
		self.color = color


	def price(self):
		print("One {} tree is 100 dollars".format(self.name))


class Pear(Tree):
	"""docstring for ClassName"""
	def __init__(self, name, age, color):
		super().__init__(name,age)
		self.color = color

	def price(self):
		print("One {} {} tree is 80 dollars".format(self.color,self.name))
		

	def grow(self):
		#重写父类的方法grow生长
		print("it was {} years old,now it is {} years old".format(self.age,int(self.age)+1))


if __name__ == '__main__':
	a = Apple('White Apple',8,'white')
	a.grow()
	a.seed()
	a.price()
	b = Pear('Pear',8,'red')
	b.price()
	b.grow()
>>>
#White Apple tree is growing...now it's 8 years old
#White Apple tree is seeding...
#One White Apple tree is 100 dollars
#One red Pear tree is 80 dollars
#it was 8 years old,now it is 9 years old

2.类Tree是各种树的总称，可理解为苹果树，梨子树等抽象后的上层概念。类Apple和类Pear都继承了类Tree，其中self.name和self.age是父类Tree的属性，self.color是子类Apple和Pear的属性。通过继承，Apple和Pear可以直接使用父类的方法和属性。

3.父类方法的改写，可通过在子类中重写相同函数名的方式进行改写。

class Tree:
	...
	...
	def grow(self):
		print("{} tree is growing...now it's {} years old".format(self.name,self.age))
		
class Pear(Tree):
	···
	···
	def grow(self):
		#重写父类的方法grow生长
		print("it was {} years old,now it is {} years old".format(self.age,int(self.age)+1))

4.super函数的用法

• python2和python3中的区别

python2 super(Tree,self).__init__(name,age)
python3 super().__init__(name,age)

在子类中使用super()函数，super()函数可以见到理解为代表父类Tree()。当需要在子类中调用父类的方法时，我们可以用super()来表示父类Tree（）

···
···
class orange(Tree):
	def grow(self):
		#调用父类Tree的方法grow
		super().grow()
		#直接调用Tree类的方法
		Tree('Apple','16').grow()
		#重写父类方法grow
		print("{} tree has changed...".format(self.name))


orange('orange','15')
>>>
#orange tree is growing...now it's 15 years old
#Apple tree is growing...now it's 16 years old
#orange tree has changed...		

• Python允许多重继承例如class fruits(Apple,Tree)的用法，多重继承容易出现父类方法多次调用的问题，可以通过super的使用来确保父类方法只被调用一次。python类的继承顺序是根据MRO，通过C3算法来计算的。我的理解当出现类方法的多次调用时，super会确保执行距离子类最近的所继承的父类的方法。对该问题感兴趣，可以深入了解砖石继承和super。总之，不是必须一般不推荐使用多重继承