No list returned by random.shuffle()

Question

I wrote some code in two different ways which I thought were exacly equivalent but I got two very differnt answers:

This first way worked exactly as expected:

test_listA = list(range(0, 5))
random.shuffle(test_listA)
print(test_listA)

It printed out:

[2, 1, 0, 3, 4]

But when I reorganised it like so:

test_listB = random.shuffle(list(range(0, 5)))
print(test_listB)

It prints out:

None

Why the difference?

Those two are not the same. In the second case, you assign the result of the function call to the variable, but the function returns `None`. Generally, it is convention for functions or methods that mutate to return `None` (true most of the time with some exceptions like `list.pop`) — juanpa.arrivillaga, Jan 29 '20 at 14:55
@CeliusStingher This question is downvoted because it already has an answer on SO since 2013. — keepAlive, Jan 29 '20 at 14:59

Yam Mesicka · Answer 1 · 2020-01-29T15:04:56.610

1

The problem

random.shuffle returns None and shuffles the original list in place. From the documentation:

Shuffle the sequence x in place.

Assigning the value that random.shuffle returns (None) back to the shuffled list would set the value of the variable to None.

You can copy the object and work on the copy:

import copy
import random


a = [1, 2, 3, 4]
b = copy.copy(a)
random.shuffle(b)

print(f"a = {a}")
print(f"b = {b}")

a = [1, 2, 3, 4]

b = [1, 3, 4, 2]

edited Jan 29 '20 at 15:04

answered Jan 29 '20 at 14:50

Yam Mesicka

Could you please add a possible solution aswell? :D – Celius Stingher Jan 29 '20 at 14:50
2

@CeliusStingher, question contains solution – Olvin Roght Jan 29 '20 at 14:50
Thanks for it @Infinity, now it's crystal clear. +1 – Celius Stingher Jan 29 '20 at 14:52
Well, the issue is that it *returns `None`*, since the OP already knows it works in place. It is a convention that a function which mutates its input to return `None` but that isn't necessarily true – juanpa.arrivillaga Jan 29 '20 at 14:52
Thanks for your feedback. I've edited the answer accordingly :) – Yam Mesicka Jan 29 '20 at 14:58