python repeat list elements in an iterator

Question

Is there any way to create an iterator to repeat elements in a list certain times? For example, a list is given:

color = ['r', 'g', 'b']

Is there a way to create a iterator in form of itertools.repeatlist(color, 7) that can produce the following list?

color_list = ['r', 'g', 'b', 'r', 'g', 'b', 'r']

Did you actually try reading the itertools docs before you asked the question? — Padraic Cunningham, Aug 31 '16 at 08:29
@PadraicCunningham: note that the other question does *full cycle repeats*; so the OP here would need to be content with multiples of 3. — Martijn Pieters, Aug 31 '16 at 08:29
@MartijnPieters, http://stackoverflow.com/a/10325689/2141635, that is pretty much identical to your own answer. — Padraic Cunningham, Aug 31 '16 at 08:32
@PadraicCunningham: right, I missed that one. Yet the questions are sufficiently different here, and technically that answer is wrong for that question. — Martijn Pieters, Aug 31 '16 at 08:33
Does this answer your question? [Circular list iterator in Python](https://stackoverflow.com/questions/23416381/circular-list-iterator-in-python) — Tomerikoo, Dec 30 '21 at 19:56

Martijn Pieters · Accepted Answer · 2023-03-25T14:09:28.553

You can use itertools.cycle() together with itertools.islice() to build your repeatlist() function:

from itertools import cycle, islice

def repeatlist(it, count):
    return islice(cycle(it), count)

This returns a new iterator; call list() on it if you must have a list object.

Demo:

>>> from itertools import cycle, islice
>>> def repeatlist(it, count):
...     return islice(cycle(it), count)
...
>>> color = ['r', 'g', 'b']
>>> list(repeatlist(color, 7))
['r', 'g', 'b', 'r', 'g', 'b', 'r']

youkaichao · Answer 2 · 2020-01-01T11:32:33.513

1

The documentation of cycle says:

Note, this member of the toolkit may require significant auxiliary storage (depending on the length of the iterable).

I'm curious why python doesn't provide a more efficient implementation:

def cycle(it):
    while True:
        for x in it:
            yield x

def repeatlist(it, count):
    return [x for (i, x) in zip(range(count), cycle(it))]

This way, you don't need to save a whole copy of the list. And it works if list is infinitely long.

edited Jan 01 '20 at 11:32

answered Jan 01 '20 at 11:12

youkaichao

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Because `itertools.cycle` accepts an `iterable` which might be a generator that can only be generated through once, for example a `map` object. Once it's exhausted, you can't get element from it anymore. Thus your `cycle` will never reach `yield x` again, which makes it an infinite-loop. Try `print(*islice(cycle(map(int, "123")), 20))` – Escape0707 Aug 29 '20 at 09:45

python repeat list elements in an iterator

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