# python - Transpose list of lists

ID : 10055

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### Top 5 Answer for python - Transpose list of lists

92

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:

1. 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])`.
2. 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`.
3. `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.

83

One way to do it is with NumPy transpose. For a list, a:

``>>> import numpy as np >>> np.array(a).T.tolist() [[1, 4, 7], [2, 5, 8], [3, 6, 9]] ``

Or another one without zip:

``>>> map(list,map(None,*a)) [[1, 4, 7], [2, 5, 8], [3, 6, 9]] ``

77

Equivalently to Jena's solution:

``>>> l=[[1,2,3],[4,5,6],[7,8,9]] >>> [list(i) for i in zip(*l)] ... [[1, 4, 7], [2, 5, 8], [3, 6, 9]] ``

62

just for fun, valid rectangles and assuming that m[0] exists

``>>> m = [[1,2,3],[4,5,6],[7,8,9]] >>> [[row[i] for row in m] for i in range(len(m[0]))] [[1, 4, 7], [2, 5, 8], [3, 6, 9]] ``

60

Methods 1 and 2 work in Python 2 or 3, and they work on ragged, rectangular 2D lists. That means the inner lists do not need to have the same lengths as each other (ragged) or as the outer lists (rectangular). The other methods, well, it's complicated.

## the setup

``import itertools import six  list_list = [[1,2,3], [4,5,6, 6.1, 6.2, 6.3], [7,8,9]] ``

## method 1 — `map()`, `zip_longest()`

``>>> list(map(list, six.moves.zip_longest(*list_list, fillvalue='-'))) [[1, 4, 7], [2, 5, 8], [3, 6, 9], ['-', 6.1, '-'], ['-', 6.2, '-'], ['-', 6.3, '-']] ``

`six.moves.zip_longest()` becomes

The default fillvalue is `None`. Thanks to @jena's answer, where `map()` is changing the inner tuples to lists. Here it is turning iterators into lists. Thanks to @Oregano's and @badp's comments.

In Python 3, pass the result through `list()` to get the same 2D list as method 2.

## method 2 — list comprehension, `zip_longest()`

``>>> [list(row) for row in six.moves.zip_longest(*list_list, fillvalue='-')] [[1, 4, 7], [2, 5, 8], [3, 6, 9], ['-', 6.1, '-'], ['-', 6.2, '-'], ['-', 6.3, '-']] ``

## method 3 — `map()` of `map()` — broken in Python 3.6

``>>> map(list, map(None, *list_list)) [[1, 4, 7], [2, 5, 8], [3, 6, 9], [None, 6.1, None], [None, 6.2, None], [None, 6.3, None]] ``

This extraordinarily compact @SiggyF second alternative works with ragged 2D lists, unlike his first code which uses numpy to transpose and pass through ragged lists. But None has to be the fill value. (No, the None passed to the inner map() is not the fill value. It means there is no function to process each column. The columns are just passed through to the outer map() which converts them from tuples to lists.)

Somewhere in Python 3, `map()` stopped putting up with all this abuse: the first parameter cannot be None, and ragged iterators are just truncated to the shortest. The other methods still work because this only applies to the inner map().

## method 4 — `map()` of `map()` revisited

``>>> list(map(list, map(lambda *args: args, *list_list))) [[1, 4, 7], [2, 5, 8], [3, 6, 9]]   // Python 2.7 [[1, 4, 7], [2, 5, 8], [3, 6, 9], [None, 6.1, None], [None, 6.2, None], [None, 6.3, None]] // 3.6+ ``

Alas the ragged rows do NOT become ragged columns in Python 3, they are just truncated. Boo hoo progress.