iteration for loop python [duplicate]

Question

Why is this happening?

Version 1:

mylist= [1,2,3,4,5]
print mylist
for index, value in enumerate(mylist):
    value = 0
print mylist

Version 2:

mylist= [[1],[2],[3],[4],[5]]
print mylist
for index, value in enumerate(mylist):
    value[0] = 0
print mylist

Output 1:

[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5]

Output 2:

[[1], [2], [3], [4], [5]]
[[0], [0], [0], [0], [0]]

I assumed that both versions would make a local variable and do not overwrite the list itself. I guess that is not the case in second version. I'm using python 2.7. Clearly, I can have another variable inside of for loop that makes another copy of the value. It just took me some time to figure this out and it was messing up my code functionality.

Solution:

value = list(value)

Drastically Improve Your Python: Understanding Python's Execution Model — user395760
– user395760, Commented Mar 17, 2013 at 18:15

Martijn Pieters · Accepted Answer · 2013-03-17 22:45:29Z

4

You are not assigning to value in the second version, you are assigning to an index contained in the list referred to by value.

Python names are references to values, not memory locations. Some of those values are mutable, such as a list. A list contains references to other values, so value is a list and value[0] is a reference contained in the list.

To change the list in the first example, use the index from enumerate to alter that index in the list:

for index, value in enumerate(somelist):
    somelist[index] = 0

If you did not want the nested lists in your second example to change, you should make a copy of them instead:

for index, value in enumerate(somelist):
    value = value[:]  # full list slice, same as a copy.

edited Mar 17, 2013 at 22:45

answered Mar 17, 2013 at 18:14

Martijn Pieters

1.1m326 gold badges4.2k silver badges3.4k bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

Barmaley Over a year ago

So what would be the best way to create new local variable? I tried reassigning value inside second version for loop but it still changes values in the list

Barmaley Over a year ago

How do I need to address it if I dont want to change the list? I just want temporary local variable that stores value of the value (Sorry for the confusion)

Xymostech Over a year ago

@Barmaley Make a copy of the list? value = list(value)

Barmaley Over a year ago

@Xymostech I tried and I get TypeError: 'list' object is not callable

Fiver Over a year ago

@Barmaley That is because you are using the name 'list' for your variable, but list is a reserved word in Python, the method name used to create a list. Change your variable name to something like my_list = [[1],[2],[3],[4],[5]]

|

kiriloff · Accepted Answer · 2013-03-19 05:37:26Z

0

Here is another pythonic way to local copy a list:

mylist= [[1],[2],[3],[4],[5]]
print mylist
mylst = []
for index, value in enumerate(mylist):
    mylst += [[value[0]]]
print mylst

answered Mar 19, 2013 at 5:37

kiriloff

26.5k40 gold badges163 silver badges235 bronze badges

Collectives™ on Stack Overflow

iteration for loop python [duplicate]

2 Answers 2

6 Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

Comments

Linked

Related