Question

I am performing shuffle operations on nested lists in Python 3. I wish to reshuffle previously shuffled lists indefinitely until the order of the nested lists meet a specific criteria. random.shuffle operates in place and calling random.shuffle() on a previously shuffled list does not reshuffle it. What is the best way to reshuffle a list indefinitely until it meets a condition. For example, I was trying something like this, but making a new list then shuffling it doesn’t seem to work:

from random import shuffle

L1 = [[1,2], [3,4], [5,6], [7,8], [9,10]]
shuffle(L1)
match = L1[0]

# reshuffle until [9,10] is the first item in the list
if match != [9,10]:
    L1 = list(L1)
    shuffle(L1)
print(L1)

Answer 1

This statement:

calling random.shuffle() on a previously shuffled list does not reshuffle it

This is incorrect. Observe:

Python 2.7.1 (r271:86832, Jun 16 2011, 16:59:05) 
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> L1 = [[1,2], [3,4], [5,6], [7,8], [9,10]]
>>> from random import shuffle
>>> shuffle(L1)
>>> print L1
[[3, 4], [9, 10], [5, 6], [7, 8], [1, 2]]
>>> shuffle(L1)
>>> print L1
[[9, 10], [3, 4], [5, 6], [1, 2], [7, 8]]
>>> 

The following code should do what you want, although it will have a non-deterministic runtime.

from random import shuffle

L1 = [[1,2], [3,4], [5,6], [7,8], [9,10]]
match = [9,10]
while L1[0]!=match:
    shuffle(L1)
print(L1)