Python __init__、super方法
转至:https://blog.csdn.net/brucewong0516/article/details/79121179
1、继承
- 在Python中,同时支持单继承与多继承,一般语法如下:
class SubClassName(ParentClass1 [, ParentClass2, ...]): class_suite
实现继承之后,子类将继承父类的属性,也可以使用内建函数insubclass()来判断一个类是不是另一个类的子孙类:
# -*- coding: utf-8 -*- """ Created on Sun Jan 21 22:33:09 2018 @author: BruceWong """ class Parent(object): ''' parent class ''' numList = [] def numdiff(self, a, b): return a-b class Child(Parent): pass c = Child() # subclass will inherit attributes from parent class #子类继承父类的属性 Child.numList.extend(range(10)) print(Child.numList) print("77 - 2 =", c.numdiff(77, 2)) # built-in function issubclass() print(issubclass(Child, Parent)) print(issubclass(Child, object)) # __bases__ can show all the parent classes #bases属性查看父类 print('the bases are:',Child.__bases__) # doc string will not be inherited #doc属性不会被继承 print(Parent.__doc__) print(Child.__doc__)
代码的输出为:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 77 - 2 = 75 True True the bases are: (<class '__main__.Parent'>,) parent class None
例子中唯一特别的地方是文档字符串。文档字符串对于类,函数/方法,以及模块来说是唯一的,也就是说doc属性是不能从父类中继承来的。
2、继承中的_ _init_ _
当在Python中出现继承的情况时,一定要注意初始化函数_init_的行为:
- 如果子类没有定义自己的初始化函数,父类的初始化函数会被默认调用;但是如果要实例化子类的对象,则只能传入父类的初始化函数对应的参数,否则会出错。
- 如果子类定义了自己的初始化函数,而在子类中没有显示调用父类的初始化函数,则父类的属性不会被初始化
- 如果子类定义了自己的初始化函数,在子类中显示调用父类,子类和父类的属性都会被初始化
2.1、子类没有定义自己的初始化函数,父类的初始化函数会被默认调用:
#定义父类:Parent class Parent(object): def __init__(self, name): self.name = name print("create an instance of:", self.__class__.__name__) print("name attribute is:", self.name) #定义子类Child ,继承父类Parent class Child(Parent): pass #子类实例化时,由于子类没有初始化,此时父类的初始化函数就会默认被调用 #且必须传入父类的参数name c = Child("init Child")
子类实例化时,由于子类没有初始化,此时父类的初始化函数就会默认被调用,此时传入父类的参数name,输出结果为:
create an instance of: Child name attribute is: init Child
如果不传入父类的参数name:
class Parent(object): def __init__(self, name): self.name = name print("create an instance of:", self.__class__.__name__) print("name attribute is:", self.name) class Child(Parent): pass #c = Child("init Child") #print() c = Child()
没有传入父类name参数的输出结果会报错:
Traceback (most recent call last): File "<ipython-input-11-9a7781a6f192>", line 1, in <module> runfile('C:/Users/BruceWong/.spyder-py3/类的继承.py', wdir='C:/Users/BruceWong/.spyder-py3') File "C:\Anaconda3\lib\site-packages\spyder\utils\site\sitecustomize.py", line 866, in runfile execfile(filename, namespace) File "C:\Anaconda3\lib\site-packages\spyder\utils\site\sitecustomize.py", line 102, in execfile exec(compile(f.read(), filename, 'exec'), namespace) File "C:/Users/BruceWong/.spyder-py3/类的继承.py", line 54, in <module> c = Child() TypeError: __init__() missing 1 required positional argument: 'name'
2.2、子类定义了自己的初始化函数,而在子类中没有显示调用父类的初始化函数,则父类的属性不会被初始化
class Parent(object): def __init__(self, name): self.name = name print("create an instance of:", self.__class__.__name__) print("name attribute is:", self.name) #子类继承父类 class Child(Parent): #子类中没有显示调用父类的初始化函数 def __init__(self): print("call __init__ from Child class") #c = Child("init Child") #print() #将子类实例化 c = Child() print(c.name)
在子类中没有显示调用父类的初始化函数,则父类的属性不会被初始化,因而此时调用子类中name属性不存在:
AttributeError: ‘Child’ object has no attribute ‘name’ call __init__ from Child class Traceback (most recent call last): File "<ipython-input-12-9a7781a6f192>", line 1, in <module> runfile('C:/Users/BruceWong/.spyder-py3/类的继承.py', wdir='C:/Users/BruceWong/.spyder-py3') File "C:\Anaconda3\lib\site-packages\spyder\utils\site\sitecustomize.py", line 866, in runfile execfile(filename, namespace) File "C:\Anaconda3\lib\site-packages\spyder\utils\site\sitecustomize.py", line 102, in execfile exec(compile(f.read(), filename, 'exec'), namespace) File "C:/Users/BruceWong/.spyder-py3/类的继承.py", line 56, in <module> print(c.name) AttributeError: 'Child' object has no attribute 'name'
2.3、如果子类定义了自己的初始化函数,显示调用父类,子类和父类的属性都会被初始化
class Parent(object): def __init__(self, name): self.name = name print("create an instance of:", self.__class__.__name__) print("name attribute is:", self.name) class Child(Parent): def __init__(self): print("call __init__ from Child class") super(Child,self).__init__("data from Child") #要将子类Child和self传递进去 #c = Child("init Child") #print() d = Parent('tom') c = Child() print(c.name)
子类定义了自己的初始化函数,显示调用父类,子类和父类的属性都会被初始化的输出结果:
#实例化父类Parent的结果 create an instance of: Parent name attribute is: tom #实例化子类Child的结果 call __init__ from Child class #super首先会先使得父类初始化的参数进行实例化 create an instance of: Child name attribute is: data from Child data from Child
3、super的使用详解
- super主要来调用父类方法来显示调用父类,在子类中,一般会定义与父类相同的属性(数据属性,方法),从而来实现子类特有的行为。也就是说,子类会继承父类的所有的属性和方法,子类也可以覆盖父类同名的属性和方法。
class Parent(object): Value = "Hi, Parent value" def fun(self): print("This is from Parent") #定义子类,继承父类 class Child(Parent): Value = "Hi, Child value" def ffun(self): print("This is from Child") c = Child() c.fun() c.ffun() print(Child.Value)
输出结果:
This is from Parent This is from Child Hi, Child value
但是,有时候可能需要在子类中访问父类的一些属性,可以通过父类名直接访问父类的属性,当调用父类的方法是,需要将”self”显示的传递进去的方式:
class Parent(object): Value = "Hi, Parent value" def fun(self): print("This is from Parent") class Child(Parent): Value = "Hi, Child value" def fun(self): print("This is from Child") Parent.fun(self) #调用父类Parent的fun函数方法 c = Child() c.fun()
输出结果:
This is from Child This is from Parent #实例化子类Child的fun函数时,首先会打印上条的语句,再次调用父类的fun函数方法
这种方式有一个不好的地方就是,需要经父类名硬编码到子类中,为了解决这个问题,可以使用Python中的super关键字:
class Parent(object): Value = "Hi, Parent value" def fun(self): print("This is from Parent") class Child(Parent): Value = "Hi, Child value" def fun(self): print("This is from Child") #Parent.fun(self) super(Child,self).fun() #相当于用super的方法与上一调用父类的语句置换 c = Child() c.fun()
输出结果:
This is from Child
posted on 2020-10-26 16:40 gentleman_hai 阅读(660) 评论(0) 编辑 收藏 举报