Non nested for loops in a single line for creating list in python

Question

I am a novice in python. I wanted to create a list in a single line:

>>> x = [[-1, y] for y in range(-1, 2), [0, 1], [1, z] for z in range(1, -2, -1), [0, -1]]

to get :

>>>> x = [[-1, -1], [-1, 0], [-1, 1], [0, 1], [1, 1], [1, 0], [1, -1], [0, -1]]

but it gave error : name 'z' is not defined. So I had to do it with for loops and single pairs on separate lines, appending each time. Before that I did :

>>> x = [[-1, y] for y in range(-1, 2),[0, 1]]
>>> x
[[-1, [-1, 0, 1]], [-1, [0, 1]]]

which tells me y is taken as iterator of for loop as well as [0, 1]. I am wrongly corelating comma in C and python. How do I achieve what I intend to do (in a single line) ? (I know that I could do it directly since its a small range. But what if there was a bigger one?)

Is the order random on purpose? I initially thought you meant to perform [ [y,z] for y in range(-1, 2) for z in range(-1, 2)]. Please explain yourself better. — Zach P
– Zach P, Commented May 17, 2015 at 13:36
A list comprehension produces a homogenous list. You are trying to create a heterogenous list, one that contains both lists of ints (e.g. [-1, -1]) and lists of lists of ints ([[1,1], [1,0], [1, -1]]). — chepner
– chepner, Commented May 17, 2015 at 13:54

Yuri Kovalev · Accepted Answer · 2015-05-17 13:56:21Z

2

You should concatenate lists, and put nested list to brackets

>>> [[-1, y] for y in range(-1, 2)] + [[0, 1], [[1, z] for z in range(1, -2, -1)], [0, -1]]
[[-1, -1], [-1, 0], [-1, 1], [0, 1], [[1, 1], [1, 0], [1, -1]], [0, -1]]

answered May 17, 2015 at 13:56

Yuri Kovalev

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skolsuper · Accepted Answer · 2015-05-17 14:09:22Z

Check out itertools. Not sure from your question whether you're going to want product, permutations or combinations but I think one of those will be what you need.

edit: On closer inspection, you're doing something much simpler and you just missed a few brackets. Try:

x = [[[-1, y] for y in range(-1, 2)], [0, 1], [[1, z] for z in range(1, -2, -1)], [0, -1]]

List comprehensions go in their own set of brackets, it's similar to the syntax of a list literal but not the same.

edit 2:

Before that I did :
x = [[-1, y] for y in range(-1, 2),[0, 1]]
x [[-1, [-1, 0, 1]], [-1, [0, 1]]]
which tells me y is taken as iterator of for loop as well as [0, 1]. I am wrongly corelating comma in C and python. How do I achieve what I intend to do (in a single line) ? (I know that I could do it directly since its a small range. But what if there was a bigger one?)

Yes you're misunderstanding what your comma is doing there. It's turning the 2nd part of your for loop into a tuple of (range(-1,2), [0, 1]) (2 items). What you needed to do here was close your list comprehension after range(-1,2), and then wrap the whole thing in square brackets to make it a literal list declaration:

x = [[[-1, y] for y in range(-1, 2)],[0, 1]]

As mentioned by @YuriyKovalev, it would probably be clearer to concat the lists like so:

x = [[-1, y] for y in range(-1, 2)] + [0, 1]

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