Python 类继承，__bases__, __mro__, super
Python是面向對象的編程語言，也支持類繼承。
>>> class Base:
... pass
...
>>> class Derived(Base):
... pass
??
這樣就定義了兩個類，Derived繼承了Base。issubclass(a,b)能夠測試繼承關系：
>>> issubclass(Derived, Base)
True
??
在Python中，每一個類有一個__bases__屬性，列出其基類
>>> Derived.__bases__
(<class '__main__.Base'>,)
??
同C++，Python支持多重繼承；
>>> class Derived2(Derived,Base):
... pass
...
Derived2繼承了Derived和Base（實際中不會這么寫）
>>> Derived2.__bases__
(<class '__main__.Derived'>, <class '__main__.Base'>)
??
這里，Derived,和Base的順序不能搞反
>>> 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++
| C++代碼 |
| class Base{ }; ?? class Derived: public Base{ }; ?? class Derived2: public Base, public Derived{ }; ?? int main(){ } |
| mulit_inherit.cc:7:7: warning: direct base 'Base' inaccessible in 'Derived2' due to ambiguity [enabled by default] class Derived2: public Base, public Derived{ ^ |
| mulit_inherit.cc:7:7: warning: direct base 'Base' inaccessible in 'Derived2' due to ambiguity [enabled by default] class Derived2: public Derived, public Base{ ^ |
能夠見，C++并沒有限制書寫順序。warning指示了Derrived2中不能訪問Base
Derived2 d;
Base &b = d;
error: 'Base' is an ambiguous base of 'Derived2'
Base &b = d;
^
??
回到Python繼承，Derived2是Derived的子類，也是Base的子類
>>> issubclass(Derived2, Base)
True
>>> issubclass(Derived2, Derived)
True
??
__bases__類似于Javascript中Object對象的__proto__，是實現繼承的基礎，不同在于：__bases__不可改動，并且__bases__是類的屬性而不是對象屬性（Javascript是基于對象的語言）；
??
>>> 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'>)
??
對象的__class__屬性指明了所屬類型；
>>> [].__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，是對象還是基本類型？
??
__mro__
__mro__給出了method resolution order，即解析方法調用的順序。
??
>>> 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，僅僅是最后面多了個<class 'object'>
??
super
super函數能夠用于調用父類的方法，而后者可能被子類覆蓋；類似在java中的作用，但使用起來更復雜些。
>>> 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
代理對象并非類層次中某個類的實例！
??
結合多重繼承來理解下__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__)
執行結果
| enter __init__@A exit __init__@A (<class '__main__.A'>, <class 'object'>) enter __init__@B enter __init__@A exit __init__@A exit __init__@B (<class '__main__.B'>, <class '__main__.A'>, <class 'object'>) enter __init__@C enter __init__@A exit __init__@A exit __init__@C (<class '__main__.C'>, <class '__main__.A'>, <class 'object'>) enter __init__@D enter __init__@A exit __init__@A exit __init__@D (<class '__main__.D'>, <class '__main__.A'>, <class 'object'>) enter __init__@E enter __init__@B enter __init__@C enter __init__@A exit __init__@A exit __init__@C exit __init__@B exit __init__@E (<class '__main__.E'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <class 'object'>) enter __init__@F enter __init__@E enter __init__@B enter __init__@C enter __init__@D enter __init__@A exit __init__@A exit __init__@D exit __init__@C exit __init__@B exit __init__@E exit __init__@F (<class '__main__.F'>, <class '__main__.E'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.D'>, <class '__main__.A'>, <class 'object'>) |
觀察到，super的運行路徑和類的__mro__列舉的類順序吻合；而__mro__的順序能夠看作是深搜的結果
A
/ | \
B C D
\ / /
E /
\ /
F
class E(B, C)中，B和C不是基-派生類關系，E.__mro__中B在C之前，須要注意；
??
多態
??
>>> class Base:
... def sayHello(self):
... print("Base says hello")
...
>>> class Derived(Base):
... pass
...
>>> d = Derived()
>>> d.sayHello()
Base says hello
Derived重寫sayHello
>>> class Derived(Base):
... def sayHello(self):
... print("Derived says hello");
...
>>> d = Derived()
>>> d.sayHello()
Derived says hello
??
與參數默認值結合（聯想起了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，在基類構造函數中調用被覆蓋方法的行為
>>> 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
可見，行為類似Java，調用了子類的覆蓋方法；
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