Flatten the list in one line (challenge)

Question

Here's a little challenge:

Convert this:
list = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]
to this:
[1, 2, 3, 4, 5, 6, 7, 8, 9]
with one line of code.
(not counting the first line, which is list = [[1, 2, 3], [4, 5, 6], [7], [8, 9]])

Answer 1

Here it is
https://code.sololearn.com/ciUEXT6W62Lb/?ref=app

Answer 2

VcC has the one I was thinking of but Tim's works fine too!

Answer 3

print([element for sublist in list for element in sublist])

is shorthand for

newlist = []
for sublist in list:
for element in sublist:
newlist.append(element)
print(newlist)

https://code.sololearn.com/c8AzvmAQv5yc/#py

Answer 4

Another way is simply:

print(sum(list, [])

but this is apparently slower.

Answer 5

@VcC your cool code suggests that it is not slower. I got that idea from https://mathieularose.com/how-not-to-flatten-a-list-of-lists-in-JUMP_LINK__&&__python__&&__JUMP_LINK/

Answer 6

Another one

https://code.sololearn.com/cIJahh32SNpZ/?ref=app

Answer 7

@Eric sum() is just a built-in function that adds the elements of an iterable.

E.g.
this_list = [1, 2, 3, 4]
print(sum(this_list))
# outputs 10

sum(list, []) adds (concatenates) the lists in 'list'

so in the challenge example, print(sum(sum(Iist, []))) yields 45

Answer 8

@VcC kindly explain your code for me

Answer 9

I've got oneliner here!
https://code.sololearn.com/c1f6tbapyHL1/?ref=app

Answer 10

You take each element from each sib list of the main list, and make a list out of this !

Answer 11

Why isit slower?

Answer 12

Don't see such a big difference, see here. In fact the sum version looks faster
https://code.sololearn.com/cHxS7O7vr4nD/?ref=app

Answer 13

The difference is 100% to 200% ( with an improved version of chrono which deducts the duration of the call to an empty function) - so the sum version is indeed slower... What is funny is that the multiple line naive version with a for loop is much faster ! Interesting article.

Answer 14

https://code.sololearn.com/cHxS7O7vr4nD/?ref=app