無法修改for循環內的列表項目

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我發現在執行for循環顯示可變項目列表時，我無法修改該項目，但是如果該元素是可變的，我可以修改該項目的元素。爲什麼？無法修改for循環內的列表項目

# Create alist which contain mutable 
alist = [[1, 2, 3],] 

#1 For loop 
for x in alist: 
    x = x + [9,] 
print alist 
# let's replace alist[0] to list which contain another one and try to modify it 
alist[0] = [[[1,2],3]] 

print2 alist # [[[[1, 2], 3]]] 
#2 For loop 
for x in alist: 
    x[0] = x[0] + [9,] 
# list modified ... 
print alist # [[[[1, 2], 3, 9]]], Modified !

我知道修改你所迭代的名單是不是好的做法（這是更好地遍歷它的副本），所以請不要點我的那一刻。

來源

2016-03-07 Andriy Ivaneyko

看一看此http： //stackoverflow.com/questions/2022031/python-append-vs-operator-on-lists-why-do-these-give-different-results。 – AKS

@AKS謝謝，請看看它 –

這樣的行爲的原因是，雖然用於環路＃執行新列表是通過將操作者+列出產生（也可以通過獲取的xid跟蹤）。

但是在for循環＃2通過項目的訪問元素與索引x[0]已在內存中的對象正在被修改。它可以通過調用X id很容易被發現，而迭代alist

# Create alist which contain mutable 
alist = [[1, 2, 3],] 

#1 for loop 
for x in alist: 
    print id(x), id(alist[0]), 'Same?:', id(x) == id(alist[0]) 
    # id(x): 4425690360 id(alist[0]) 4425690360 Same?: True 
    x = x + [9,] 
    print id(x), id(alist[0]), 'Same?:', id(x) == id(alist[0]) 
    # id(x): 4425770696 id(alist[0]) 4425690360 Same?: False 
# mutable item not modified, so my guess is that x actually are copy of that item 
print alist # [[1, 2, 3]] 
# let's replace alist[0] to list which contain another one and try to modify it 
alist[0] = [[[1,2],3]] 

print alist 
#2 for loop 
for x in alist: 
    print id(x), id(alist[0]), 'Same?:', id(x) == id(alist[0]) 
    # id(x): 4425811008 id(alist[0]) 4425811008 Same?: True 

    x[0] = x[0] + [9,] 
    print id(x), id(alist[0]), 'Same?:', id(x) == id(alist[0]) 
    # id(x): 4425811008 id(alist[0]) 4425811008 Same?: True 
# list modified ... 
print alist # [[[[1, 2], 3, 9]]]

要解決的問題使用array.append內進行循環修改項目，請參閱下面的示例：

alist = [[1, 2, 3],] 
for x in alist: 
    print id(x), id(alist[0]), 'Same?:', id(x) == id(alist[0]) 
    x.append(9) 
    print id(x), id(alist[0]), 'Same?:', id(x) == id(alist[0]) 
# list modified ... 
print alist # [[1, 2, 3, 9],]

來源

2016-03-07 03:36:26

無法修改for循環內的列表項目

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