python inheritance note

Reference

http://www.artima.com/weblogs/viewpost.jsp?thread=281127

http://apt-blog.net/constructors_with_different_arguments_in_python_multiinheri_class

Python Inheritance 繼承

繼承型態如下

class DerivedClassName(BaseClassName): <statement-1> . . . <statement-N>

舊式繼承

class Father: def __init__(self,f_name,f_age): self.f_name = f_name self.f_age = f_age def f_info(self): print("papa name:",self.f_name,"papa age:",self.f_age) class Child(Father): def __init__(self,c_name,c_age,f_name,f_age): Father.__init__(self,f_name,f_age) self.c_name = c_name self.c_age = c_age self.f_info() def fam_info(self): print("age sum:",self.f_age+self.c_age)

python 3以後可以用super來繼承,父類繼承object才能使用super函式,即使是多重繼承也只需要呼叫一次super()

class Father(object): def __init__(self,f_name,f_age): self.f_name = f_name self.f_age = f_age def f_info(self): print("papa name:",self.f_name,"papa age:",self.f_age) class Child(Father): def __init__(self,c_name,c_age,f_name,f_age): super(Child,self).__init__(f_name,f_age) self.c_name = c_name self.c_age = c_age self.f_info() def fam_info(self): print("age sum:",self.f_age+self.c_age)

>>> d = Child('mary',10,'john',40)

('papa name:', 'john', 'papa age:', 40)

>>> d.fam_info()

('age sum:', 50)

Multi Inheritance 多重繼承

繼承順序是由左到右

舊式

class Father: def __init__(self,f_name,f_age): self.f_name = f_name self.f_age = f_age def f_info(self): print("papa name:",self.f_name,"papa age:",self.f_age) class Mother: def __init__(self,m_name,m_age): self.m_name = m_name self.m_age = m_age def m_info(self): print("mama name:",self.m_name,"mama age:",self.m_age) class Child(Father,Mother): def __init__(self,c_name,c_age,f_name,f_age,m_name,m_age): Father.__init__(self,f_name,f_age) Mother.__init__(self,m_name,m_age) self.c_name = c_name self.c_age = c_age self.f_info() self.m_info() def fam_info(self): print("age sum:",self.f_age+self.m_age+self.c_age)

>>> d = Child('mary',10,'john',40,'ann',35)

('papa name:', 'john', 'papa age:', 40)

('mama name:', 'ann', 'mama age:', 35)

>>> d.fam_info()

('age sum:', 85)

新式繼承super

super在單繼承跟多繼承有兩種不同的工作方式

將父類別繼承母類別,再由子類別繼承父類別來呼叫父母類別的函式

class Mother(object): def __init__(self,m_name,m_age): self.m_name = m_name self.m_age = m_age def m_info(self): print("mama name:",self.m_name,"mama age:",self.m_age) class Father(Mother): def __init__(self,f_name,f_age,m_name,m_age): super(Father,self).__init__(m_name,m_age) self.f_name = f_name self.f_age = f_age def f_info(self): print("papa name:",self.f_name,"papa age:",self.f_age) class Child(Father,Mother): def __init__(self,c_name,c_age,f_name,f_age,m_name,m_age): super(Child,self).__init__(f_name,f_age,m_name,m_age) self.c_name = c_name self.c_age = c_age self.f_info() self.m_info() def fam_info(self): print("age sum:",self.f_age+self.m_age+self.c_age)

如果使用super有多重分支,會走上最上層的父繼承,再走另一分支

因為他在多重繼承裡使用MRO (Method Resolution Order)

MRO是深度優先的繼承列表

class A(object): def __init__(self): super(A,self).__init__() print "A" class B(A): def __init__(self): super(B,self).__init__() print "B" class C(A): def __init__(self): super(C,self).__init__() print "C" class D(object): def __init__(self): super(D,self).__init__() print "D" class E(B,C,D): def __init__(self): super(E,self).__init__() print "E"

>>> d = E()

D

A

C

B

E

>>>

上例如果沒有super做多重繼承的話,必須找出繼承順序寫在子類別中

class A(object): def __init__(self): print "A" class B(A): def __init__(self): print "B" class C(A): def __init__(self): print "C" class D(object): def __init__(self): print "D" class E(B,C,A,D): def __init__(self): D.__init__(self) A.__init__(self) C.__init__(self) B.__init__(self) print "E"

>>> d = E()

D

A

C

B

E