# Pairwise crossproduct in Python

## Question:

How can I get the list of cross product *pairs* from a list of arbitrarily long lists in Python?

## Example

```
a = [1, 2, 3]
b = [4, 5, 6]
```

`crossproduct(a,b)`

should yield `[[1, 4], [1, 5], [1, 6], ...]`

.

## Answers:

You’re looking for itertools.product if you’re on (at least) Python 2.6.

```
>>> import itertools
>>> a=[1,2,3]
>>> b=[4,5,6]
>>> itertools.product(a,b)
<itertools.product object at 0x10049b870>
>>> list(itertools.product(a,b))
[(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)]
```

Since you asked for a list:

```
[(x, y) for x in a for y in b]
```

But you can avoid the overhead of a list if you’re just looping through these by using generators instead:

```
((x, y) for x in a for y in b)
```

Behaves identically in a `for`

loop but doesn’t result in the creation of a `list`

.

Using generators there is no need for itertools, simply:

```
gen = ((x, y) for x in a for y in b)
for u, v in gen:
print u, v
```