更改一个字典值会更改所有值

Question

我在Python Dictionary中看到一些不寻常的行为:

import numpy as np
td =[np.Inf, 2, 3]
a = {}
# First initialize contents of dictionary to a list of values
for k in range(10):
    a[k] = td

#now I want to access the contents to modify them based on certain criteria 
for k in range(10):
    c = a[k]
    c[0] = k
    a[k] = c

从这里我可以预期每个字典键值的列表中的每个第一项都会根据(c [0] = k)进行更改,但是,我最后得到的是字典的所有值都更新到最后k的值:如

{0: [9, 2, 3], 1: [9, 2, 3], 2: [9, 2, 3], 3: [9, 2, 3], 
 4: [9, 2, 3], 5: [9, 2, 3], 6: [9, 2, 3], 7: [9, 2, 3], 
 8: [9, 2, 3], 9: [9, 2, 3]}

我错过了什么,或者字典定义中有什么问题？我可以以不同的方式解决这个问题,以便我的代码能够运行,但我对字典类的行为方式感兴趣.

Answer 1

因为每个键都获得相同的列表 ...要创建列表的浅表副本,请使用以下语法:

for k in range(10):
    a[k] = td[:] 

演示:

>>> d = {}
>>> el = [1, 2, 3]
>>> d[0] = el
>>> d[1] = el
>>> map(id, d.values())
[28358416, 28358416]