Python: Is there a one-liner to create a list with repeated elements?

Question

I can create a list that has repeated elements of another list as follows:

xx = ["a","b"]
nrep = 3
print xx
yy = []
for aa in xx:
 for i in range(nrep):
 yy.append(aa)
print yy

output:

['a', 'b']
['a', 'a', 'a', 'b', 'b', 'b']

Is there a one-liner to create a list with repeated elements?

Answer 1

yy = [aa for aa in xx for _ in range(nrep)]

I suggest that you try this sort of the thing at an interactive prompt, it's a great way to learn.

You might also want to take a look at the itertools module https://docs.python.org/3/library/itertools.html.

Answer 2

Sure. As with all one liners, there comes a degree of complexity when it gets in the way of readability; you must decide what is better in your use case.

Look up the chain() function from the itertools module. Generate 2 (or nrep) length lists from each element of the original list and chain() them together. That gets you an iterable of all the elements. If you really need a list out the end instead of the iterable of the elements, convert the iterable to a list (hint: lists can be initialized with iterables).

Answer 3

yy = reduce(lambda a, b: a + b, ([i] * nrep for i in xx), [])

Or, if you want to "import operator" first, you can use 'operator.add' instead of the lambda (but you _did_ ask for a one-liner ;)).