Transpose list of lists

Question

Let's take: ``` l = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] ``` The result I'm looking for is ``` r = [[1, 4, 7], [2, 5, 8], [3, 6, 9]] ``` and not ``` r = [(1, 4, 7), (2, 5, 8), (3, 6, 9)] ```

Accepted Answer

Python 3:

```
# short circuits at shortest nested list if table is jagged:
list(map(list, zip(*l)))

# discards no data if jagged and fills short nested lists with None
list(map(list, itertools.zip_longest(*l, fillvalue=None)))
```

Python 2:

```
map(list, zip(*l))
```

```
[[1, 4, 7], [2, 5, 8], [3, 6, 9]]
```

Explanation:

There are two things we need to know to understand what's going on:

- The signature of zip: `zip(*iterables)` This means `zip` expects an arbitrary number of arguments each of which must be iterable. E.g. `zip([1, 2], [3, 4], [5, 6])`.

- Unpacked argument lists: Given a sequence of arguments `args`, `f(*args)` will call `f` such that each element in `args` is a separate positional argument of `f`.

- `itertools.zip_longest` does not discard any data if the number of elements of the nested lists are not the same (homogenous), and instead fills in the shorter nested lists then zips them up.

Coming back to the input from the question `l = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]`, `zip(*l)` would be equivalent to `zip([1, 2, 3], [4, 5, 6], [7, 8, 9])`. The rest is just making sure the result is a list of lists instead of a list of tuples.

Transpose list of lists

Solution

Problem