Python - Iterating of any type of list structure

Question

Sorry if this has already been answered, I'm very new to python, I've had a good look around and have found this page here which has helped me a little but i'm still stuck.

I'm trying to get any type of input to work in my script, I've got it working for a single item and a list of items, but i'm finding it hard to get it to work for a list of lists.

I've edited the code as per comments to make a little more sense:

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

if isinstance(Input, list):
    Input = Input
else:
    Input = [Input]

listout = []


for x in Input:
    listout.append(x+2)

print (listout)

returns: line 12, in listout.append(x+2) TypeError: can only concatenate list (not "int") to list

This works if Input = 1 or Input = [1,2,3,4] for example, but not for the above.

I would like the output to look like the below for a list of lists:

[[3,4,5],[6,7,8],[9,10,11]]

I attempted to make a flat list out of the nested lists first but i'd like to keep the list structure for the output.

Thanks all for reading,

TJ

maybe you could use pre-defined Inputs instead of using some library for that. Maybe just Input = 1, Input = [1,2,3], etc. and then explain based on these what doesn't work (or what you want). — MSeifert
– MSeifert, Commented Aug 31, 2017 at 14:00
I recommend to avoid writing code that attempts to handle inhomogeneous input in the first place. It results in ugly code. — wim
– wim, Commented Aug 31, 2017 at 14:50
@wim so you recommend having two separate scripts? One for nested lists and one for a single input/list? — TJames
– TJames, Commented Aug 31, 2017 at 14:57

dawg · Accepted Answer · 2017-08-31 16:20:23Z

2

You might consider numpy:

>>> import numpy as np
>>> a=np.array([[1,2,3],[4,5,6],[7,8,9]])
>>> a
array([[1, 2, 3],
       [4, 5, 6],
       [7, 8, 9]])
>>> a+=2
>>> a
array([[ 3,  4,  5],
       [ 6,  7,  8],
       [ 9, 10, 11]])
>>> a.tolist()
[[3, 4, 5], [6, 7, 8], [9, 10, 11]]

If you cannot use numpy, you will need to write a recursive procedure for arbitrary nesting:

def nested_sum(e,n):
    if isinstance(e, list):
        return [nested_sum(x, n) for x in e]
    else:
        return e+n    

>>> nested_sum([1,[2,3],[4,5,6],[7,[8],9]], 2)
[3, [4, 5], [6, 7, 8], [9, [10], 11]]

If you just have two levels of nesting (as you have in the example), you can do a list comprehension:

>>> li=[[1,2,3],[4,5,6],[7,8,9]]
>>> [[e+2 for e in sl] for sl in li]
[[3, 4, 5], [6, 7, 8], [9, 10, 11]]

edited Aug 31, 2017 at 16:20

answered Aug 31, 2017 at 14:38

dawg

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3 Comments

TJames Over a year ago

thanks this does work, but I'm using this in a programme which uses a .NET based ironpython implementation so it doesn't support numpy (to the best of my knowledge).

dawg Over a year ago

Updated. Try the recursive or list comprehension.

TJames Over a year ago

Cheers @dawg you're a star, the nested_sum definition is exactly what I was looking for.

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