Python values of multiple lists in one list comprehension

Question

Is there any smart way to write a list comprehension over more than one list?

I know I could use a separate range list as index but this way I have to know the length (or get it separately with a len() function call).

>>> a = range(10)
>>> b = range(10, 0, -1)
>>> [(a[x],b[x]) for x in range(10)]
[(0, 10), (1, 9), (2, 8), (3, 7), (4, 6), (5, 5), (6, 4), (7, 3), (8, 2), (9, 1)]

I'd love to have something like this:

>>> [(a,b) for a in range(10) and b in range(10, 0, -1)]
[(0, 10), (1, 9), (2, 8), (3, 7), (4, 6), (5, 5), (6, 4), (7, 3), (8, 2), (9, 1)]

How would you write the list comprehension? Is there a way to do this with itertools?

The range list just stand for any list and I do not necessarily want to get a tuple. there could also be a function which takes a and b as parameters. So zip is not what I want.

UPDATE: With "So zip is not what I want." I meant that I don't want zip(range(10), range(10, 0, -1))

Duplicate of 9184497?

quazgar
– quazgar

2013-04-08 07:30:43 +00:00
Commented Apr 8, 2013 at 7:30 — quazgar
– quazgar, Commented Apr 8, 2013 at 7:30

agf · Accepted Answer · 2012-03-22 16:53:12Z

20

Your example is just:

zip(range(10), range(10, 0, -1))

More generally, you can join any set of iterables using zip:

[func(a, d, ...) for a, b, ..., n in zip(iterable1, iterable2, ..., iterableN)]

answered Mar 22, 2012 at 16:53

agf

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3 Comments

dominik Over a year ago

The second solution is what I want. Thanks a lot.

D.Shawley Over a year ago

You might want to consider itertools.izip since it is better at large sequences. Same idea, just a slightly different tool.

Ryan J. Smith Over a year ago

I think you might've meant to write [func(a, b, ...) for a, b, ..., n in zip(iterable1, iterable2, ..., iterableN)]

Marcin · Accepted Answer · 2012-03-22 18:55:21Z

1

If you want to apply a function to several sequences, you need either map or itertools.imap:

map(lambda *x: sum(x), range(10), range(10, 0, -1), range(0,20, 2))

There is no need to zip unless you prefer to do your mapping in a list comprehension

answered Mar 22, 2012 at 18:55

Marcin

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6 Comments

agf Over a year ago

That's not what he wants. Look at his second example -- he wants to zip. And you never need map, a list comprehension / generator expression can do everything it can and more.

Marcin Over a year ago

@agf " there could also be a function which takes a and b as parameters. So zip is not what I want." You could argue that one also never needs zip, because map is available.

agf Over a year ago

He wrote that before I added my second example, showing how to use a function with zip. Read his comment on my answer written after I added it. He meant he didn't want just zip, not that he didn't want zip at all.

Marcin Over a year ago

@agf Why zip and apply a function when you can do it in one step?

agf Over a year ago

Because a list comprehension is more flexible and Pythonic. You can write the expression as a function, but you don't have to. You can use the included filtering expression rather than having to use a separate filter step.

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