Iteration over list slices

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If you want to divide a list into slices you can use this trick:

list_of_slices = zip(*(iter(the_list),) * slice_size)

For example

>>> zip(*(iter(range(10)),) * 3)
[(0, 1, 2), (3, 4, 5), (6, 7, 8)]

If the number of items is not dividable by the slice size and you want to pad the list with None you can do this:

>>> map(None, *(iter(range(10)),) * 3)
[(0, 1, 2), (3, 4, 5), (6, 7, 8), (9, None, None)]

It is a dirty little trick

OK, I’ll explain how it works. It’ll be tricky to explain but I’ll try my best.

First a little background:

In Python you can multiply a list by a number like this:

[1, 2, 3] * 3 -> [1, 2, 3, 1, 2, 3, 1, 2, 3]
([1, 2, 3],) * 3 -> ([1, 2, 3], [1, 2, 3], [1, 2, 3])

And an iterator object can be consumed once like this:

>>> l=iter([1, 2, 3])
>>> l.next()
1
>>> l.next()
2
>>> l.next()
3

The zip function returns a list of tuples, where the i-th tuple contains the i-th element from each of the argument sequences or iterables. For example:

zip([1, 2, 3], [20, 30, 40]) -> [(1, 20), (2, 30), (3, 40)]
zip(*[(1, 20), (2, 30), (3, 40)]) -> [[1, 2, 3], [20, 30, 40]]

The * in front of zip used to unpack arguments. You can find more details here.
So

zip(*[(1, 20), (2, 30), (3, 40)])

is actually equivalent to 

zip((1, 20), (2, 30), (3, 40))

but works with a variable number of arguments

Now back to the trick:

list_of_slices = zip(*(iter(the_list),) * slice_size)

iter(the_list) -> convert the list into an iterator

(iter(the_list),) * N -> will generate an N reference to the_list iterator.

zip(*(iter(the_list),) * N) -> will feed those list of iterators into zip. Which in turn will group them into N sized tuples. But since all N items are in fact references to the same iterator iter(the_list) the result will be repeated calls to next() on the original iterator

I hope that explains it. I advice you to go with an easier to understand solution. I was only tempted to mention this trick because I like it.

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