[英]Rebinding immutable class attribute in the base class from a derived class
我在python中有這個通用問題。 基類定義一個類屬性class_attr
。 此屬性是不可變的,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
重新綁定到新值(在我的玩具箱中,將其遞增)。
問題是如何在不顯式命名Base.class_attr += 1
Base
情況下執行此操作。
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 :我很想問這個問題,也就是我可以重新綁定父類的屬性嗎? 我沒有解決此問題的解決方法(例如,將increment_class_attr
移至Base)。
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
如果您有多個繼承:
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
假設您也不知道繼承順序,則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
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
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