How to unpack a list and assign it to variable for further use(Python)?

Question

x=[[1.0, -1.0, 0.0, 3.0], [1.0, 2.0, 3.0, 4.0], ...]

x is a nested list

Expected o/p:

x1=[1.0, -1.0, 0.0, 3.0]
x2=[[1.0, 2.0, 3.0, 4.0]

further i will be using x1, x2, ... separately in different functions. E.g.

x_add=x1+x2

I tried using loop but was not able to get expected o/p I want the loop to iterate up to length of x and extract each sub list from list x and assign it to a variable xn iteratively.

Do you know how many variables you're trying to assign (i.e. the length of the list `x`)? — Ari Cooper-Davis, Aug 23 '21 at 16:06
Welcome to SO! Only unpack the list if it's 2-3 variables. Anything more than that or if the number of elements are unknown and you want to unpack all of them, [keep it a list](https://stackoverflow.com/questions/1373164/how-do-i-create-variable-variables). See [unpack the first two elements in list/tuple](https://stackoverflow.com/questions/11371204/unpack-the-first-two-elements-in-list-tuple) — ggorlen, Aug 23 '21 at 16:08

Ari Cooper-Davis · Answer 1 · 2021-08-27T11:41:19.607

1

If you know the length of list x you can assign its contents to variables like so:

x = [[1, 2, 3], [4, 5, 6]]
x1, x2 = x
print(x1) # [1, 2, 3]
print(x2) # [4, 5, 6]

If you don't know the length of list x you're almost certainly better off not trying to approach your problem in this way! So, having warned you that this really isn't a good idea, you can do it using exec():

x = [[1, 2, 3], [4, 5, 6]]
for i, _x, in enumerate(x):
   exec(f"x{i+1} = _x")

print(x1) # [1, 2, 3]
print(x2) # [4, 5, 6]

This is a significant security risk, will make your code very hard to read and debug, is not a good use of variable types, and should absolutely be avoided.

edited Aug 27 '21 at 11:41

answered Aug 23 '21 at 16:09

Ari Cooper-Davis

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Thanks for answering. actually X varies as per user input and i need to assign Xn values to n variables – code_flipper Aug 23 '21 at 16:10
Later on in your code how will you know whether `x5` has been defined or not? Can you explain what you're trying to acheive? – Ari Cooper-Davis Aug 23 '21 at 16:15
I want the loop to iterate up to length of X and extract each sub list from list X and assign it to a variable Xn iteratively – code_flipper Aug 23 '21 at 16:19
I understand that, I'm saying that this is going to make the rest of your code very difficult. It's like asking a plumber to remove the taps in your house - they'll understand what you're asking them to do, but tell you it's not a good idea. Why are you trying to do that? If you're removing your taps because they're too hard to turn then your plumber is better off fixing that instead! – Ari Cooper-Davis Aug 23 '21 at 16:21
lol I just wanna extract sub list.., can you be a good plumber and suggest some alternative. – code_flipper Aug 23 '21 at 16:30
But *why*? What do you want to do with the sub lists? Without knowing that all I can suggest for how to get the 3rd sublist in `x` is to use `x[2]`... – Ari Cooper-Davis Aug 23 '21 at 16:31
I have mentioned above have a look – code_flipper Aug 23 '21 at 16:34
Do you mean `x_add = x1 + x2`? You could do this instead `x_add = x[0] + x[1]`. – Ari Cooper-Davis Aug 23 '21 at 16:39
what about x[n-1]+xn? – code_flipper Aug 23 '21 at 17:21
`x[5-1] + x[5]`. I'm done with this question, read the documentation: https://docs.python.org/3/tutorial/datastructures.html – Ari Cooper-Davis Aug 23 '21 at 17:27
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I'm done with your answers – code_flipper Aug 24 '21 at 16:39

How to unpack a list and assign it to variable for further use(Python)?

1 Answers1