How to declare an array using all elements of another array?

Question

Assume I have a "constant" like:

ITEMS = [ "item1", "item2" ]

Now I would like to create an array that contains all entries of ITEMS, plus some more.

Obviously, this is wrong:

more_things = [ "all", ITEMS ]

as it would put the whole array into the new array. But I want the new array to contain [ "all", "item1", "item2" ] in the end.

Sure, I could somehow iterate the first array, but I am wondering if there is a more idiomatic one liner way to do this. Ideally, it should work for python2 and python3.

meowgoesthedog · Accepted Answer · 2019-02-13 09:54:58Z

2

Just concatenate them with +:

more_things = ["all"] + ITEMS

answered Feb 13, 2019 at 9:54

meowgoesthedog

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3 Comments

meowgoesthedog Over a year ago

@GhostCat yep, works for both major versions of Python; I didn't downvote, but IMO people are more likely to downvote this for the abundance of duplicates than its simplicity.

meowgoesthedog Over a year ago

@GhostCat StackOverflow works in mysterious ways, but if I had to guess it could be fear of retribution

GhostCat Over a year ago

Ah, well. I really dont mind simple (correct) close as dup votes. If there would have been one early on,I just delete the question and move on. But comment-less downvotes, that sucks. And I think hope assume that I dont have a reputation of going revenging things ...

Eugene Yarmash · Accepted Answer · 2019-02-13 13:06:15Z

2

If you're running Python 3.5+, the closest to your request is to use unpacking:

>>> ITEMS = ['item1', 'item2']
>>> ['all', *ITEMS]
['all', 'item1', 'item2']

In earlier versions list concatenation would do the same:

>>> ITEMS = ['item1', 'item2']
>>> ['all'] + ITEMS
['all', 'item1', 'item2']

If you simply need an iterable (i.e. not necessarily a list), using itertools.chain() may be more efficient:

from itertools import chain
more_things = chain(['all'], ITEMS)  # or wrap in list() to get a list

edited Feb 13, 2019 at 13:06

answered Feb 13, 2019 at 9:56

Eugene Yarmash

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Comments

javidcf · Accepted Answer · 2019-02-13 10:01:42Z

1

In Python 3 you can do:

ITEMS = [ "item1", "item2" ]
more_things = [ "all", *ITEMS ]

This is quite flexible because it lets you combine several lists (or iterables in general) and elements in one line:

even_more_things = [ "all", *ITEMS, "the", *ITEMS, "items" ]

answered Feb 13, 2019 at 10:01

javidcf

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Comments

cs95 · Accepted Answer · 2019-02-13 10:06:35Z

1

I'll suggest one more obvious approach: list.insert.

ITEMS.insert(0, 'all')
ITEMS
# ['all', 'item1', 'item2']

It modifies ITEMS in-place.

If you don't want to modify ITEMS, you can create a copy... although at that point, you'd rather one of the simpler options like iterable unpacking (*)...

ITEMS2 = ITEMS.copy()
ITEMS2.insert(0, 'all')

ITEMS2
# ['all', 'item1', 'item2']

edited Feb 13, 2019 at 10:06

answered Feb 13, 2019 at 10:03

cs95

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1 Comment

GhostCat Over a year ago

ITEMS is supposed to be a "constant". more_things on the other hand is supposed to go into a choices array for argparse. So my question is really about creating a new array, without changing the first one. But sure, it is worth noting that changing is possible, too.

Mehrdad Pedramfar · Accepted Answer · 2019-02-13 10:01:50Z

0

You can also use itertools.chain

from itertools import chain

more_things = list(chain(more_things, ITEMS))

it is useful when you want to add more than two lists together.

answered Feb 13, 2019 at 10:01

Mehrdad Pedramfar

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