Get random sample from list while maintaining ordering of items?

Question

I have a sorted list, let say: (its not really just numbers, its a list of objects that are sorted with a complicated time consuming algorithm)

mylist = [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ,9  , 10 ]

Is there some python function that will give me N of the items, but will keep the order?

Example:

randomList = getRandom(mylist,4)
# randomList = [ 3 , 6 ,7 , 9 ]
randomList = getRandom(mylist,4)
# randomList = [ 1 , 2 , 4 , 8 ]

etc...

Answer 1

Following code will generate a random sample of size 4:

import random

sample_size = 4
sorted_sample = [
    mylist[i] for i in sorted(random.sample(range(len(mylist)), sample_size))
]

(note: with Python 2, better use xrange instead of range)

Explanation

random.sample(range(len(mylist)), sample_size)

generates a random sample of the indices of the original list.

These indices then get sorted to preserve the ordering of elements in the original list.

Finally, the list comprehension pulls out the actual elements from the original list, given the sampled indices.