Python初始化类中的列表 [英] Python's initialization a list in a class

问题描述

我不了解python的行为，我有很多相同的错误:

I don't understand python's behavior, I've got many the same errors:

print2中的文件"./start.py"，第20行，
item.print2()
RuntimeError:超过最大递归深度

File "./start.py", line 20, in print2
item.print2()
RuntimeError: maximum recursion depth exceeded

我的代码如下:

#!/usr/bin/python

class Component:
    def print2(self):
        pass

class Composite(Component):
    items = []
    name = 'composite'

    def __init__(self, name):
        self.name = name

    def add(self, item):
        self.items.append(item)

    def print2(self):
        if self.items:
            for item in self.items:
                item.print2()
        else:
            print self.name

class Leaf(Component):
    name = 'leaf'

    def __init__(self, name):
        self.name = name

    def print2(self):
        print self.name

category = Composite('category1')
leaf = Composite('leaf1')
category.add(leaf)
leaf = Leaf('leaf2')
category.add(leaf)

category.print2()

当我在构造函数( __ init __ )中添加 self.items = [] 时，它可以正常工作.您能解释一下这种行为吗?

When I add self.items = [] in the constructor ( __init__ ), it works fine. Could you explain the behavior?

推荐答案

items 是一个类变量，因此它由 Composite 的每个实例共享.当您将 append 附加到 items 中时，所有实例都会获得该项目.因此，添加 leaf 后，您将拥有:

items is a class variable, so it is shared by every instance of Composite. When you append into items, all instances get that item. So after adding the leafs, you have:

   Type                    Composite.items
------------------------------------------
 Composite    category1    [leaf1, leaf2]
                  |
 Composite      leaf1      [leaf1, leaf2]
                  |
      Leaf      leaf2

如果在 leaf1 上调用了 print2 ，它将尝试在其中的每个 items 上调用 print2 该类，它一次又一次地无限次地包含 leaf1 .

If print2 is called on leaf1, it will try to call print2 on each of the items in the class, which includes leaf1 again, and again, in an infinite recursion.

如果您在 __ init __ 内部初始化 self.items = [] ，它将是一个实例变量而不是类变量，因此每个 Composite 将具有其自己的 items 列表.

If you instead initialize self.items = [] inside __init__, it will be an instance variable instead of a class variable, so each Composite will have its own list of items.

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