從派生類中重新綁定基類中的不可變類屬性

Question

我在python中有這個一般問題。基類定義了一個類屬性class_attr。這個屬性是不可變的，在這種情況下它是一個數字。我想從派生類中更改此屬性，從而將Base.class_attr重新綁定爲新值（在我的玩具案例中，將其遞增）。

問題是如何在沒有明確命名Base的語句Base.class_attr += 1中這樣做。

class Base(object): 
    # class attribute: 
    class_attr = 0 

class Derived(Base): 
    @classmethod 
    def increment_class_attr(cls):   
     Base.class_attr += 1 
     # is there a solution which does not name the owner of the 
     # class_attr explicitly? 

     # This would cause the definition of Derived.class_attr, 
     # thus Base.class_attr and Derived.class_attr would be 
     # two independent attributes, no more in sync: 
    # cls.class_attr += 1 

Derived.increment_class_attr() 
Derived.increment_class_attr() 

print Base.class_attr # 2 

請注意：我是非常的問題後，也就是我可以重新綁定一個父類的屬性。我並沒有解決這個問題的解決方法（例如，將increment_class_attr轉移到基地）。

Answer 1

使用__bases__屬性：

In [68]: class Base(object): 
    ...:  # class attribute: 
    ...:  class_attr = 0 
    ...:  

In [69]: class Derived(Base): 
    ...:  @classmethod 
    ...:  def inc(cls): 
    ...:   p, = cls.__bases__ 
    ...:   p.class_attr += 1 
    ...:   

In [70]: Base.class_attr 
Out[70]: 0 

In [71]: Derived.inc() 

In [72]: Derived.inc() 

In [73]: Base.class_attr 
Out[73]: 2 

如果你有多重繼承：

In [88]: class DifferentInherited(object): 
    ...:  class2_attr = 0 
    ...: 


In [90]: class Der2(Base, DifferentInherited): 
    ...:  @classmethod 
    ...:  def inc(cls): 
    ...:   print cls.__bases__ 
    ...:   a, b, = cls.__bases__ 
    ...:   print a.class_attr 
    ...:   print b.class2_attr 
    ...:   

In [91]: Der2.inc() 
(<class '__main__.Base'>, <class '__main__.DifferentInherited'>) 
2 
0 

假設你不知道繼承順序要麼，你就需要測試每個類的變量：

In [127]: class Der3(DifferentInherited, Base): 
    ...:  @classmethod 
    ...:  def inc(cls): 
    ...:   # This gets a list of *all* classes with the attribute `class_attr` 
    ...:   classes = [c for c in cls.__bases__ if 'class_attr' in c.__dict__] 
    ...:   for c in classes: 
    ...:    c.class_attr += 1 
    ...:    

In [128]: Der3.inc() 

In [129]: Base.class_attr 
Out[129]: 3 

In [131]: DifferentInherited.class2_attr 
Out[131]: 0 

和多重繼承使用__mro__：

In [146]: class Multi(Der3): 
    ...:  @classmethod 
    ...:  def inc(cls): 
    ...:   c_attr = [c for c in cls.__mro__ if 'class_attr' in c.__dict__] 
    ...:   print c_attr 
    ...:   c_attr[0].class_attr += 1 
    ...:   

In [147]: Multi.inc() 
[<class '__main__.Base'>] 

In [148]: Base.class_attr 
Out[148]: 4