Substring function appears not to work in lists

[Jason de Vere]

Can someone explain why this won't work?

my_list = ["bananas", "apples", "pears"]

substring = "anana"

for fruit in my_list:
	if substring in fruit:
		fruit = fruit.replace(substring, "alloon")

print(my_list)

output: ['bananas', 'apples', 'pears']

The list remains unchanged, even though it is a mutable type. It works fine it it's not in a list:

text = "bananas"

substring = "anana"

if substring in text:
	text = text.replace(substring, "alloon")

print(text)

output: ['balloons', 'apples', 'pears']

Thanks
Jason

[Piet Souris]

Strings are immutable.

Your array contains pointers to the strings. In your for-loop, fruit is just another pointer to the corresponding string (it is a copy of the original pointer, this is called 'passing by value'). So, although you change that fruit-pointer, the original pointer in your array (pointing to "bananas") remains unchanged. Can you now explain why text does get changed?

[Jason de Vere]

Sorry, what I meant was that the list is mutable, since that is what I was changing. The string was meant to be totally replaced with the line: fruit = fruit.replace(substring, "alloon")

I didn't really understand your explanation but it did help me see that I never actually touched the list. I would have said I changed a copy of one of the list's values, but I'm sure that's not correct either. My solution would be:

my_list = ["bananas", "apples", "pears"]
 
substring = "anana"
 
for fruit in my_list:
    if substring in fruit:
        i = my_list.index(fruit)
        my_list[i] = my_list[i].replace(substring, "alloon")    

print(my_list)

Thanks for the help.