如何在python中的for循环中更改列表元素的值？

Question

I am having a bit of trouble changing the values of list elements when utilizing a for loop. The following code outputs 10 and 5 - but I'm expecting 10 and 10.

amount = 5
costFieldList = [amount]

for index, field in enumerate(costFieldList):
    if type(field) is int:
        costFieldList[index] = field*2
        print(costFieldList[index])
print(amount)

Is this an issue of scope? Thanks in advance.

Answer 1

You are printing amount at the end. This is set to an immutable value (5).

If your last line is print(costFieldList) , you will see that it is [10] as expected. You used amount to initialize the list, but there is no link back to modify amount.

Answer 2

By writing costFieldList[index] = field*2 , you are creating an entirely new int instance which then overwrites the "reference" to the instance in the array, but not the value itself. So you lose the reference to the original value of amount .

At this stage amount still has a "reference" to 5 which is why you get 5 as the second output.

Answer 3

If you want to set both amount and costFieldList[index] to field * 2 use:

amount = 5
costFieldList = [amount]

for index, field in enumerate(costFieldList):
    if isinstance(field,int):  # use issinstance to check if field is an integer
        costFieldList[index] = field * 2
        print(costFieldList)
        amount=costFieldList[index] # set amount to value of costFieldList index 
print(amount,costFieldList ) 
(10, [10])