Pick N distinct items at random from sequence of unknown length, in only one iteration

If your sequence is short enough that reading it into memory and randomly sorting it is acceptable, then a straightforward approach would be to just use random.shuffle:

import random
arr=[1,2,3,4]

# In-place shuffle
random.shuffle(arr)

# Take the first 2 elements of the now randomized array
print arr[0:2]
[1, 3]

Depending upon the type of your sequence, you may need to convert it to a list by calling list(your_sequence) on it, but this will work regardless of the types of the objects in your sequence.

Naturally, if you can't fit your sequence into memory or the memory or CPU requirements of this approach are too high for you, you will need to use a different solution.

Use reservoir sampling. It's a very simple algorithm that works for any N.

Here is one Python implementation, and here is another.

Simplest I've found is this answer in SO, improved a bit below:

import random

my_list = [1, 2, 3, 4, 5]
num_selections = 2

new_list = random.sample(my_list, num_selections)

# To preserve the order of the list, you could do:
randIndex = random.sample(range(len(my_list)), n_selections)
randIndex.sort()
new_list = [my_list[i] for i in randIndex]