[英]Weird Python behaviour - or am I missing something
The following code: 以下代码:
class House:
links = []
class Link:
pass
class Villa(House):
pass
if __name__ == '__main__':
house = House()
villa = Villa()
link = Link()
house.links.append(link)
print house.links
print villa.links
results in this output: 结果输出:
[<__main__.Link instance at 0xb65a4b0c>]
[<__main__.Link instance at 0xb65a4b0c>]
I find this very weird: Since it is another instance? 我觉得这很奇怪:既然是另一个例子? - I would have expected that the output is - Since it is another instance?:
- 我原以为输出是 - 因为它是另一个实例?:
[<__main__.Link instance at 0xb65a4b0c>]
[]
When changing the line house.links.append(link)
to house.links = [link]
everything works as expected. 当更改行
house.links.append(link)
到house.links = [link]
一切都按预期工作。
Can somebody explain this behavior? 有人可以解释这种行为吗?
It is another instance, but you have defined links
as a class variable rather than an instance variable. 它是另一个实例,但您已将
links
定义为类变量而不是实例变量。
An instance variable would be defined as such: 实例变量将被定义为:
class House(object): # Always use new-style classes except for backward compatibility
def __init__(self):
self.links = []
Note that in Python, unlike other languages, an instance variable is explicitly declared to be a property of an instance. 请注意,在Python中,与其他语言不同,实例变量显式声明为实例的属性。 This usually happens in the
__init__
method, to ensure that every instance has the variable. 这通常发生在
__init__
方法中,以确保每个实例都有变量。
Then, the subclass would look like this: 然后,子类看起来像这样:
class Villa(House):
def __init__(self):
super(Villa, self).__init__()
And executing your code gives the correct results: 执行代码会得到正确的结果:
>>> house = House()
>>> villa = Villa()
>>> link = Link()
>>> house.links.append(link)
>>> print house.links
[<__main__.Link instance at 0xcbaa8>]
>>> print villa.links
[]
In your code, links
is an attribute on the class, making it shared by all instances of that class: 在您的代码中,
links
是类的一个属性,使其由该类的所有实例共享:
class House:
links = []
Because of this, your Villa
class shares this attribute, because an instance of Villa
is (as a subclass) also an instance of House
. 因此,您的
Villa
类共享此属性,因为Villa
的实例(作为子类)也是House
的实例。
If you want to make links
an instance variable, write a constructor and set links
as an attribute of self
, eg 如果要将
links
作为实例变量,请编写构造函数并将links
设置为self
的属性,例如
class House:
def __init__(self):
self.links = []
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