In Python how to modify a list with a for loop?

Question

I am unsure on how to get something like this to work.

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

def convert(temp):
   for element in temp:
      element = 2
   return temp

data = convert(data)
print(data) #[1, 2, 3, 4, 5]

Something like this works as expected

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

def convert(temp):
   for i in range(len(temp)):
      temp[i] = 2
   return temp

data = convert(data)
print(data) #[2, 2, 2, 2, 2]

I am wondering if there is any way to make the first example work(the list would become just 2s)

@MauriceMeyer import to understand, that isn't modifying the list at all. — juanpa.arrivillaga, Aug 28 '21 at 20:10
Yeah, you're right, it's the same id. So it's a reference and immutable. I'll remove my post in case it was misleading. — ggorlen, Aug 28 '21 at 21:16

Samwise · Answer 1 · 2021-08-28T20:11:24.137

Obligatory: don't use list as a variable or parameter name, since it overwrites the builtin list type.

The reason the first example doesn't work is that each value from the list is assigned to the variable named element, but reassigning element to a different value only modifies that variable; it doesn't mutate the original value in the list.

The second example does what you want because the [] operator allows you to mutate the list.

Note also that there is an example between mutating the original list and returning a new list that's a modified version of the original!

>>> data = [1, 2, 3, 4, 5]
>>> def convert(a):
...     """Returns a new list filled with 2s."""
...     return [2 for i in a]
...
>>> def mutate(a):
...     """Modifies the list to fill it with 2s."""
...     for i in range(len(a)):
...         a[i] = 2
...     return a
...
>>> convert(data)
[2, 2, 2, 2, 2]
>>> data
[1, 2, 3, 4, 5]
>>> mutate(data)
[2, 2, 2, 2, 2]
>>> data
[2, 2, 2, 2, 2]

Both functions return a list that's filled with 2s, but convert returns a new list (leaving the original data unchanged unless you assign the new value to it), whereas mutate modifies the original list as well.

Finally, there is a way to use the [] operator as a shortcut to mutate the entire list at once:

>>> def mutate(a):
...     a[:] = [2 for _ in a]
...
>>> data = [1, 2, 3, 4, 5]
>>> mutate(data)
>>> data
[2, 2, 2, 2, 2]

In Python how to modify a list with a for loop?

1 Answers1