Changing first item in nested list using list comprehension

Question

Let's say I have the following list of lists:

[['Hello im an item', 1234], ['I enjoy python', 'hello'], ['I only have one item']]

I want to replace the spaces with underscores in the first item in each inner list so it looks like this:

[['Hello_im_an_item', 1234], ['I_enjoy_python', 'hello'], ['I_only_have_one_item']]

I have tried the following with no success:

formatted = [first[0].replace(' ', '_') for first in formatted]

formatted returns a list of only the first items, but with the correct formatting. Does anyone have any idea how this can be accomplished?

Answer 1

A list comprehension builds an entirely new list, so you need to ensure that that new list contains everything the old list contained. Concatenate the remainder of each sublist:

formatted = [[sub[0].replace(' ', '_')] + sub[1:] for sub in formatted]

As a regular for loop, the above is roughly equivalent to:

_new = []
for sub in formatted:
    _element = [sub[0].replace(' ', '_')] + sub[1:]
    _new.append(_element)
formatted = _new

Rather than use a list comprehension, you could just alter your list in-place with a regular loop:

for sub in formatted:
    sub[0] = sub[0].replace(' ', '_')

Only use the list comprehension if you absolutely need a completely new list.

Answer 2

Just use a for loop:

for a in formatted:
    a[0] = a[0].replace(' ', '_')

This really isn't a use-case for list comprehensions.