Understading a quicksort python implementation

Question

I found this code online.

def quick_sort(items):
""" Implementation of quick sort """
if len(items) > 1:
    pivot_index = len(items) / 2
    smaller_items = []
    larger_items = []
    for i, val in enumerate(items):
        if i != pivot_index:
            if val < items[pivot_index]:
                smaller_items.append(val)
            else:
                larger_items.append(val)

    quick_sort(smaller_items)
    quick_sort(larger_items)
    items[:] = smaller_items + [items[pivot_index]] + larger_items

The one line that gives me trouble is the last one. I believe it's basic concatenation, however, when I change "items[:]" to "items", the algorithm fails. What is special about the [:] at the end of the list?

If anyone one can help, I would really appreciate. Thank you in advance!

This is slice assignment, see e.g. stackoverflow.com/q/10623302/3001761 — jonrsharpe
– jonrsharpe, Commented Dec 7, 2014 at 9:02

Photon · Accepted Answer · 2014-12-07 08:56:48Z

2

This is in-place assignment, without replacing the list object with a new object. This way, the output is placed in the original list and can be read by the caller.

answered Dec 7, 2014 at 8:56

Photon

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3 Comments

David Tran Over a year ago

Oh wow, that's really cool. I really haven't seen this anywhere else before despite thinking that I know a lot of about python. Additionally, may I ask if this is application to dict and tuple?

Photon Over a year ago

I'm not sure. Try and see. :-)

PM 2Ring Over a year ago

No. It's not applicable to dict because dicts don't support slice indexing, and it's not applicable to tuple because tuples are immutable.

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