Python - Can you use list comprehension to replace the value at a particular list index?

Question

I currently have a bunch of lists defined something like below:

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

I'm currently replacing the first element of that list then placing the contents of the list into a dictionary (where the key is the old element 0 value) by doing the following:

old_value = old_list[0]    
old_list[0] = 'new value'
test_dict[old_value] = old_list

I was wondering, is this the best way to accomplish this? I was wondering if there was a way to make this a bit more efficient with list comprehension so it could look something a bit more like this:

test_dict[old_list[0]] = [i for idx, i in enumerate(old_list) if '''relevant conditions to replace element 0''']

Answer 1

Here is one way to do it

Code

old_list = [1, 2, 3, 4, 5]
new_value = 'new'
test_dict = {}
test_dict[old_list[0]] = [new_value] + old_list[1:]
print(test_dict)

Output

{1: ['new', 2, 3, 4, 5]}

Generalised form

old_list = [1, 2, 3, 4, 5]
idx = 2
new_value = 'new'
test_dict = {}

test_dict[old_list[idx]] = old_list[:idx] + [new_value] + old_list[idx+1:]
print(test_dict)

Output

{3: [1, 2, 'new', 4, 5]}

Answer 2

A readable form can be achieved with unpacking:

head, *tail = old_list

# if test_dict already exists
test_dict[head] = ["new value"] + tail

# otherwise
test_dict = {head: ["new value"] + tail}
# {1: ['new value', 2, 3, 4, 5]}