lime gale
Beginner question ~
If I have three lists a and b and c what are the recommended ways for creating a new list with all of the elements of a, b and c?
It looks like there's a simple a + b + c ~ but then the internet gives me complicated-looking alternatives involving .extend() and from itertools import chain.
Is there some big non-obvious downside to x = a + b + c? It seems simple and clear code to me.
(Using Python 3.12.)
wary summitBOT
Beginner question ~
If I have three lists `a` and `b` and `c` what are the rec...
@lime gale
Python help channel opened
cinder gate
If you are just wanting to create a new list then the simple + approach is all you need. If you want to modify a list then extend is an option. The itertool solutions you've found would typically be for a scenario when it needs to support combining any number of lists
The only downside to + approach is when your lists are super large and it needs to be solved in a more memory efficient manner
woven monolith
The itertools method specifically is nice when your lists are big and you don’t instantly need all the elements. Iterators are lazy, so they only get the data as needed. This can be very worthwhile if you are joining big lists, but only need a small portion at a time.
rain fiber
chaining adds also re-allocates each time
if you actually need a list as output, and not an iterator, then I suggest this syntax
joined = [*a, *b, *c]
I appreciate the answers. I'm just a beginner doing small things, so it sounds like + works fine. I've made notes for the other explanations. Thank you. 😸
urban stream
!e py a = [1, 2, 3] b = [a, a, a] print(b) a[0] = 0 print(b)
wary summitBOT
!e ```py
a = [1, 2, 3]
b = [a, a, a]
print(b)
a[0] = 0
print(b)```
:white_check_mark: Your 3.12 eval job has completed with return code 0.
001 | [[1, 2, 3], [1, 2, 3], [1, 2, 3]]
002 | [[0, 2, 3], [0, 2, 3], [0, 2, 3]]
!e py a = [[1, 2, 3]] b = [[4, 5, 6]] c = [[7, 8, 9]] d = a + b + c print(d) b[0][:] = 0, 0, 0 print(d)Beware of lists with references to mutables.
wary summitBOT
!e ```py
a = [[1, 2, 3]]
b = [[4, 5, 6]]
c = [[7, 8, 9]]
d = a + b + c
print(d)
...
:white_check_mark: Your 3.12 eval job has completed with return code 0.
001 | [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
002 | [[1, 2, 3], [0, 0, 0], [7, 8, 9]]
Concatenating and extending lists that only have references to immutables is often fine.
Lists of ints/strings, etc.
Lists of sets, lists, dictionaries, immutables that in turn have mutables...that's where you can run into trouble.
This is more a caveat of working with lists in general, rather than joining them.
wary summitBOT
Python help channel closed