来自 dict.fromkeys 的不良行为 [英] Unwanted behaviour from dict.fromkeys

问题描述

我想使用 dict.fromkeys 初始化一个集合字典(在 Python 2.6 中)，但结果结构的行为很奇怪.更具体地说:

<预><代码>>>>>x = {}.fromkeys(range(10), set([]))>>>>X{0: set([]), 1: set([]), 2: set([]), 3: set([]), 4: set([]), 5: set([]), 6: set([]), 7: set([]), 8: set([]), 9: set([])}>>>>x[5].add(3)>>>>X{0: set([3]), 1: set([3]), 2: set([3]), 3: set([3]), 4: set([3]), 5: set([3]), 6: set([3]), 7: set([3]), 8: set([3]), 9: set([3])}

我显然不想在所有集合中添加 3，只想添加到 x[5] 对应的集合中.当然，我可以通过在没有 fromkeys 的情况下初始化 x 来避免这个问题，但我想了解我在这里遗漏了什么.

解决方案

dict.fromkeys 的第二个参数只是一个值.您已经创建了一个字典，该字典将 same 设置为每个键的值.大概你了解它的工作方式:

<预><代码>>>>一个 = 集()>>>b = a>>>b.添加(1)>>>乙设置([1])>>>一种设置([1])

你在那里看到了同样的行为；在您的情况下，x[0]、x[1]、x[2](等)都是访问确切信息的不同方式相同的 set 对象.

对于字符串表示形式包括其内存地址的对象，这更容易看到，您可以在其中看到它们是相同的:

<预><代码>>>>dict.fromkeys(range(2), object()){0:<0x1001da080 处的对象对象>，1:<0x1001da080处的对象>}

I'd like to initialise a dictionary of sets (in Python 2.6) using dict.fromkeys, but the resulting structure behaves strangely. More specifically:

>>>> x = {}.fromkeys(range(10), set([]))
>>>> x
{0: set([]), 1: set([]), 2: set([]), 3: set([]), 4: set([]), 5: set([]), 6: set([]), 7: set([]), 8: set([]), 9: set([])}
>>>> x[5].add(3)
>>>> x
{0: set([3]), 1: set([3]), 2: set([3]), 3: set([3]), 4: set([3]), 5: set([3]), 6: set([3]), 7: set([3]), 8: set([3]), 9: set([3])}

I obviously don't want to add 3 to all sets, only to the set that corresponds to x[5]. Of course, I can avoid the problem by initialising x without fromkeys, but I'd like to understand what I'm missing here.

解决方案

The second argument to dict.fromkeys is just a value. You've created a dictionary that has the same set as the value for every key. Presumably you understand the way this works:

>>> a = set()
>>> b = a
>>> b.add(1)
>>> b
set([1])
>>> a
set([1])

you're seeing the same behavior there; in your case, x[0], x[1], x[2] (etc) are all different ways to access the exact same set object.

This is a bit easier to see with objects whose string representation includes their memory address, where you can see that they're identical:

>>> dict.fromkeys(range(2), object())
{0: <object object at 0x1001da080>,
 1: <object object at 0x1001da080>}

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