Python 类继承，__bases__, __mro__, super

Python是面向对象的编程语言，也支持类继承。

>>> class Base:

… pass

…

>>> class Derived(Base):

… pass

这样就定义了两个类，Derived继承了Base。issubclass(a,b)能够測试继承关系：

>>> issubclass(Derived, Base)

True

在Python中，每一个类有一个__bases__属性，列出其基类

>>> Derived.__bases__

(<class ‘__main__.Base’>,)

同C++，Python支持多重继承；

>>> class Derived2(Derived,Base):

… pass

…

Derived2继承了Derived和Base（实际中不会这么写）

>>> Derived2.__bases__

(<class ‘__main__.Derived’>, <class ‘__main__.Base’>)

这里，Derived,和Base的顺序不能搞反

>>> class Derived2(Base, Derived):

… pass

…

Traceback (most recent call last):

File “<stdin>”, line 1, in <module>

TypeError: Cannot create a consistent method resolution

order (MRO) for bases Derived, Base

插一段C++

能够见，C++并没有限制书写顺序。warning指示了Derrived2中不能訪问Base

Derived2 d;

Base &b = d;

error: ‘Base’ is an ambiguous base of ‘Derived2’

Base &b = d;

^

回到Python继承，Derived2是Derived的子类，也是Base的子类

>>> issubclass(Derived2, Base)

True

>>> issubclass(Derived2, Derived)

True

__bases__类似于Javascript中Object对象的__proto__，是实现继承的基础，不同在于：__bases__不可改动，并且__bases__是类的属性而不是对象属性（Javascript是基于对象的语言）；

>>> d = Derived2()

>>> d.__bases__

Traceback (most recent call last):

File “<stdin>”, line 1, in <module>

AttributeError: ‘Derived2’ object has no attribute ‘__bases__’

>>> d.__class__

<class ‘__main__.Derived2’>

>>> d.__class__.__bases__

(<class ‘__main__.Derived’>, <class ‘__main__.Base’>)

对象的__class__属性指明了所属类型；

>>> [].__class__

<class ‘list’>

>>> ().__class__

<class ‘tuple’>

>>> 1.__class__

File “<stdin>”, line 1

1.__class__

^

SyntaxError: invalid syntax

>>> type(1)

<class ‘int’>

在Python中1，是对象还是基本类型？

__mro__

__mro__给出了method resolution order，即解析方法调用的顺序。

>>> Derived.__mro__

(<class ‘__main__.Derived’>, <class ‘__main__.Base’>, <class ‘object’>)

>>> Derived2.__mro__

(<class ‘__main__.Derived2’>, <class ‘__main__.Derived’>, <class ‘__main__.Base’>, <class ‘object’>)

看上去和__bases__相像，仅仅是最后面多了个<class ‘object’>

super

super函数能够用于调用父类的方法，而后者可能被子类覆盖；类似在java中的作用，但使用起来更复杂些。

>>> class Base:

… pass

…

>>> class Derived(Base):

… pass

…

>>> class Derived2(Derived)

File “<stdin>”, line 1

class Derived2(Derived)

^

SyntaxError: invalid syntax

>>> class Derived2(Derived):

… pass

…

>>> d = Derived2()

>>> super(Derived2, d)

<super: <class ‘Derived2’>, <Derived2 object>>

>>> super(Derived, d)

<super: <class ‘Derived’>, <Derived2 object>>

>>> super(Base, d)

<super: <class ‘Base’>, <Derived2 object>>

參考https://docs.python.org/2/library/functions.html#super，

super(type[, object-or-type])

Return a proxy object that delegates method calls to a parent or sibling class of type. This is useful for accessing inherited methods that have been overridden in a class. The search order is same as that used by getattr() except that the type itself is skipped.

The __mro__ attribute of the type lists the method resolution search order used by both getattr() and super().

>>> isinstance(super(Base, d), Base)

False

>>> isinstance(super(Derived, d), Base)

False

代理对象并非类层次中某个类的实例！

结合多重继承来理解下__mro__和super

class A:

def __init__(self):

print(‘enter __init__@A’)

super(A,self).__init__()

print(‘exit __init__@A’)

class B(A):

def __init__(self):

print(‘enter __init__@B’)

super(B,self).__init__()

print(‘exit __init__@B’)

class C(A):

def __init__(self):

print(‘enter __init__@C’)

super(C,self).__init__()

print(‘exit __init__@C’)

class D(A):

def __init__(self):

print(‘enter __init__@D’)

super(D,self).__init__()

print(‘exit __init__@D’)

class E(B,C):

def __init__(self):

print(‘enter __init__@E’)

super(E,self).__init__()

print(‘exit __init__@E’)

class F(E,D):

def __init__(self):

print(‘enter __init__@F’)

super(F,self).__init__()

print(‘exit __init__@F’)

if __name__ == ‘__main__’:

A()

print(A.__mro__)        

B()

print(B.__mro__)        

C()

print(C.__mro__)        

D()

print(D.__mro__)        

E()

print(E.__mro__)        

F()

print(F.__mro__)

执行结果

观察到，super的运行路径和类的__mro__列举的类顺序吻合；而__mro__的顺序能够看作是深搜的结果

A

/ | \

B C D

\ / /

E /

\ /

F

class E(B, C)中，B和C不是基-派生类关系，E.__mro__中B在C之前，须要注意；

多态

>>> class Base:

… def sayHello(self):

… print(“Base says hello”)

…

>>> class Derived(Base):

… pass

…

>>> d = Derived()

>>> d.sayHello()

Base says hello

Derived重写sayHello

>>> class Derived(Base):

… def sayHello(self):

… print(“Derived says hello”);

…

>>> d = Derived()

>>> d.sayHello()

Derived says hello

与參数默认值结合（联想起了C++）

>>> class Base:

… def sayHello(self, str=”Base”):

… print(“Base says: ” + str)

…

>>> class Derived(Base):

… def sayHello(self, str=”Derived”):

… print(“Dervied says: ” + str)

…

>>>

>>> d = Derived()

>>> d.sayHello()

Dervied says: Derived

>>> Base.sayHello(d)

Base says: Base

>>> super(Derived, d).sayHello()

Base says: Base

看一下，在基类构造函数中调用被覆盖方法的行为

>>> class Base:

… def sayHello(self):

… str = self.getStr()

… print(“Base says: ” + str)

… def getStr(self):

… return “Base”

…

>>> class Derived(Base):

… def getStr(self):

… return “Derived”

…

>>>

>>> d = Derived()

>>> d.sayHello()

Base says: Derived

>>> def Base_init(self):

… self.sayHello()

…

>>> Base.__init__ = Base_init

>>> d = Derived()

Base says: Derived

可见，行为类似Java，调用了子类的覆盖方法；