chimeracoder chimeracoder - 3 months ago 31
Python Question

Create List of Single Item Repeated n Times in Python

I know a list comprehension will do this, but I was wondering if there is an even shorter (and more Pythonic?) approach.

I want to create a series of lists, all of varying length. Each list will contain the same element e, repeated n times (where n = length of the list). How do I create the lists, without doing

[e for number in xrange(n)]

for each list?


You can also write:

[e] * n

You should note that if e is for example an empty list you get a list with n references to the same list, not n independent empty lists.

Performance testing

At first glance it seems that repeat is the fastest way to create a list with n identical elements:

>>> timeit.timeit('itertools.repeat(0, 10)', 'import itertools', number = 1000000)
>>> timeit.timeit('[0] * 10', 'import itertools', number = 1000000)

But wait - it's not a fair test...

>>> itertools.repeat(0, 10)
repeat(0, 10)  # Not a list!!!

The function itertools.repeat doesn't actually create the list, it just creates an object that can be used to create a list if you wish! Let's try that again, but converting to a list:

>>> timeit.timeit('list(itertools.repeat(0, 10))', 'import itertools', number = 1000000)

So if you want a list, use [e] * n. If you want to generate the elements lazily, use repeat.