random.shuffle(list) produces empty list - why?

[HeRo]

Hi

In the sample program I have attached below I first use

SelectedTargets = random.sample(TargetPool, NoOfTargetsToSelect)

to draw out some of the items from a list. Then I want to shuffle that list with

Sessions2 = random.shuffle(SelectedTargets)

. But that gives me an empty list containing only "None". Why?

I can add that I use Python 3.5 (32-bit) through PyCharm Professional 2016.2.3 on Windows 10 64-bit.

I look forward to hear from you!

Best regards,
Henrik R.

Here is the whole program:

import random

TargetPoolFile = open('TestPool.txt', 'r')
TargetPool = TargetPoolFile.readlines()
NoOfTargetsInPool = len(TargetPool)
print("No. of targets in Pool:", NoOfTargetsInPool)
NoOfTargetsToSelect = 5
print("No. of targets to select:", NoOfTargetsToSelect)

SelectedTargets = random.sample(TargetPool, NoOfTargetsToSelect)
print(SelectedTargets)
# Until here: It works
# According to https://docs.python.org/3.5/library/random.html:
# random.shuffle(list) should rearrange the order of the items on
# the list, but instead it gives an empty list - containing "None":
Sessions2 = random.shuffle(SelectedTargets)
print(Sessions2)

Here is the contents of the input-file 'TestPool.txt':

input file

1  EUR/USD
2  AUD/USD
3  EUR/CHF
4  EUR/GBP
5  EUR/JPY
6  GBP/EUR
7  GBP/USD
8  USD/CAD
9  USD/CHF
10  USD/JPY
11  CAD/CHF
12  CAD/JPY
13  CHF/JPY
14  EUR/CAD
15  EUR/SGD
16  EUR/ZAR
17  GBP/CAD
18  GBP/CHF
19  GBP/JPY
20  GBP/SGD

[Yoriz]

random.shuffle changes the list in place instead of returning a shuffled list.

[HeRo]

random.shuffle changes the list in place instead of returning a shuffled list.

Thank you for your reply!
English is not my mother tongue, so what do you mean by "changes the list in place"?

Henrik

[Yoriz]

SelectedTargets is shuffled. The function does not return anything it alters the passed in object.

Example

import random
selected_targets = [1, 2, 3, 4, 5]
random.shuffle(selected_targets)
print(selected_targets)

Output:
[1, 2, 5, 4, 3]

If you want to keep selected_targets as it is and get a new shuffled list, you can use sample with the full list size.

target_pool = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
no_of_targets_to_select = 5
selected_targets = random.sample(target_pool, no_of_targets_to_select)
print(selected_targets)
sessions2 = random.sample(selected_targets, len(selected_targets))
print(sessions2)

Output:
[4, 7, 6, 5, 9]
[9, 4, 6, 7, 5]

[HeRo]

OK! That makes sense! Thank you very much!

- Henrik

[sparkz_alot]

I can add that I use Python 3.5 (32-bit) through PyCharm Professional 2016.2.3 on Windows 10 64-bit. 

May I ask why you are using 32 bit Python on a 64 bit operating system?

[metulburr]

May I ask why you are using 32 bit Python on a 64 bit operating system?

I do this all the time because some 3rd party libs have trouble with 64 bit libraries. (ei. python + pygame + py2exe often have problems with 64 bit when working together, and ends up being easier to just use 32 bit regardless of what the OS is)

[HeRo]

I can add that I use Python 3.5 (32-bit) through PyCharm Professional 2016.2.3 on Windows 10 64-bit. 

May I ask why you are using 32 bit Python on a 64 bit operating system?

When I followed the instructions on how/where to download Python on python.org I was led to that version.
I did notice it was 'only' 32-bit, but didn't want to spend more time on why.