Value found but another one not assigned (lists)

Question

Just the easy one which I could not find out why it works like that:

cities = ['Madrid', 'Barcelona', 'Valencia', 'Murcia']

for city in range(len(cities)):
    if cities[city] == 'Madrid':
        cities[city] = 1

    elif cities[city] == 'Barcelona':
        cities[city] = 2

    elif cities[city] == 'Valencia':
        cities[city] = 3

    else:
        cities[city] = 4

print(cities)

and this one gives [1, 2, 3, 4] as it should.

But if I do this:

cities = ['Madrid', 'Barcelona', 'Valencia', 'Murcia']
for city in cities:
    if city == 'Madrid':
        print('hey')
        city = 'Kiev'
print(cities)

Result is the following:

hey
['Madrid', 'Barcelona', 'Valencia', 'Murcia']

So, it recognizes 'Madrid', therefore prints hey, but then does not assign 'Kiev' to it. However in the first example, using index range, the reassignment is completed perfectly.

`city` is not a reference to the list element, it's a variable that holds the value that was taken out of the list. Assigning to it doesn't change the list. — Barmar, Jun 13 '22 at 21:37
You can use `for i, city in enumerate(cities)` and `city == 'whatever city here'` and `cities[i] = ...` — Eric Jin, Jun 13 '22 at 21:40
[Facts and myths about Python names and values](https://nedbatchelder.com/text/names.html) — wwii, Jun 13 '22 at 21:44
Your second piece of code *never modifies the list, or any of the objects in the list*. So you shouldn't expect anything about the list to change. — juanpa.arrivillaga, Jun 13 '22 at 21:46
But yes, I second that you should read: https://nedbatchelder.com/text/names.html — juanpa.arrivillaga, Jun 13 '22 at 21:47
"but then does not reassigns 'Kiev' to it"...does not reassign what? — Code-Apprentice, Jun 13 '22 at 21:49
@Code-Apprentice - Line 4 of the code does not reassign Kiev to the list `cities` — , Jun 13 '22 at 21:51

score 1 · Answer 1 · answered Jun 13 '22 at 22:01

Let's look at a similar piece of code:

cities = ['Madrid', 'Barcelona', 'Valencia', 'Murcia']
city = cities[0]
city = 'Kiev'
print(cities)

Output:

['Madrid', 'Barcelona', 'Valencia', 'Murcia']

Here, the final output doesn't have 'Kiev'. And we aren't surprised by this because nothing in the code modified the cities list. In your case, you have a for loop, but it is the same thing. There is no magic in the for loop that associates its variable to the original list. So changing the value of the city variable inside the for loop doesn't change the cities list.

robni · Answer 2 · 2022-06-13T21:48:00.877

0

In each iteration city will be overriden by the next item in your list cities.

If you want that Madrid will be Kiev, than try this:

for i, city in enumerate(cities):
    if city == 'Madrid':
         cities[i] = "Kiev"

edited Jun 13 '22 at 21:48

answered Jun 13 '22 at 21:39

robni

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well, to be pedantic, it is a global variable in the OP's code – juanpa.arrivillaga Jun 13 '22 at 21:46

Value found but another one not assigned (lists)

2 Answers2