Append multiple list in a list comprehension

Question

Is it possible to append multiple list during list comprehension? For example: Below works fine:

X = []
Y = []
[X.append(i) for i in range(100000)]

But, this won't even run:

[X.append(i), Y.append(i**2 + 3*i) for i in range(100000)]

How to make above work?

Why would you use a list comprehension here? It doesn't make sense. Don't use list comprehensions for side effects. — mozway, Oct 11 '22 at 18:46
**Do not use a list comprhensions for side-effects**. Never do `[X.append(i) for i in range(100000)] ` — juanpa.arrivillaga, Oct 11 '22 at 18:48
```[(X.append(i), Y.append(i**2 + 3*i)) for i in range(100000)]``` ? — Ziv Sion, Oct 11 '22 at 18:50

score 2 · Answer 1 · answered Oct 11 '22 at 18:47

You should never write the code you've written above.

[X.append(i) for i in range(...)] generates the result [None, None, None, ......] where there are 100,000 Nones in that list, and then throws it away. It just so happens that X.append(i) has a side affect that you're taking advantage of.

You're looking for

X.extend(i for i in range(100000))

And as for the code you're trying to write. Just make it two lines of code. Your readers will thank you.

Or alternatively, just X = [i for i in range(100000)] and likewise for Y.

`X = list(range(100000))` is sufficient ;) – mozway Oct 11 '22 at 18:49 — mozway, Oct 11 '22 at 18:49

score 0 · Answer 2 · answered Oct 11 '22 at 18:46

0

Direcly define your list with comprehension:

Y = [i**2 + 3*i for i in range(1000)]

answered Oct 11 '22 at 18:46

PlainRavioli

1,127
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score 0 · Answer 3 · answered Oct 11 '22 at 18:47

0

You can use zip(*...) (transposing operation) to assign to different variables:

X, Y = map(list, zip(*((i, i ** 2 + 3 * i) for i in range(10))))

print(X)
print(Y)

Prints:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[0, 4, 10, 18, 28, 40, 54, 70, 88, 108]

answered Oct 11 '22 at 18:47

Andrej Kesely

168,389
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score 0 · Answer 4 · answered Oct 11 '22 at 18:47

To answer your question, you need to wrap your two parameters in brackets to be able to do that from within list comprehension:

[(X.append(i), Y.append(i**2 + 3*i)) for i in range(100000)]

However, doing it this way is unpythonic as it will result in a list filled with None of length 10_000 which immediately gets discarded. Just use a for loop:

for i in range(10_000):
  X.append(i)
  Y.append(i**2 + 3*i)

or, two list comprehensions:

X = list(range(10_000))
Y = [i**2 for i in range(10_000)]

Append multiple list in a list comprehension

4 Answers4