奇怪的行为与python中的字典列表 [英] weird behaviour with list of dictionaries in python
问题描述
这是一个执行列表操作的简单代码:
>>> a = [0] * 5
>>>> a
[0,0,0,0,0]
>>> a [0] = 5
>>>> a
[5,0,0,0,0]
>>>>
现在,没有异常。
现在,我尝试使用一系列词典而不是整数列表:
>>> a = [{}] * 5
>>> a
[{},{},{},{},{}]
>>> a [0] ['b'] = 4
>>>> a
[{'b':4},{'b':4},{'b':4},{'b':4},{'b':4}]
>>>
我不明白为什么我的列表中的所有元素都被修改...
任何人都可以告诉我为什么?还有一个解决方法?
这不奇怪。
< >
解决方法:
a = [{} for x in xrange(5) ]
[...] * 5
创建一个 ...
和一个五个指针到这个 ...
的列表。 p>
0
是一个不可变的整数。您不能修改它,您可以用另一个整数(例如 a [0] = 5
)替换它。然后它是一个不同的整数。
{}
是一个可变字典。您正在修改它: a [0] ['b'] = 4
。它总是相同的字典。
Here is a simple code that performs operations on lists:
>>> a = [0] * 5
>>> a
[0, 0, 0, 0, 0]
>>> a[0] = 5
>>> a
[5, 0, 0, 0, 0]
>>>
For now, nothing abnormal.
Now, I try to do the same with a list of dictionaries instead of a list of integers:
>>> a = [{}] * 5
>>> a
[{}, {}, {}, {}, {}]
>>> a[0]['b'] = 4
>>> a
[{'b': 4}, {'b': 4}, {'b': 4}, {'b': 4}, {'b': 4}]
>>>
I don't understand why all elements of my list are modified...
Can anyone tell me why? And also provide a workaround?
This is not weird.
Workaround:
a = [{} for i in xrange(5)]
[…] * 5
creates one …
and a list of five pointers to this …
.
0
is an immutable integer. You cannot modify it, you can just replace it with another integer (such as a[0] = 5
). Then it is a different integer.
{}
is a mutable dictionary. You are modifying it: a[0]['b'] = 4
. It is always the same dictionary.
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