List cherry picking slicing method

Question

It's fairly simple to articulate problem, but I'm not 100% sure that I have my lingo right. Nonetheless, conceptually "cherry picking" is suitable to describe the slice I have in mind. This is because I simply want to access (cherry pick from all elements) two distant elements of a list. I tried this:

my_list[2,7]

So I was expecting it to return only 2 elements, but instead I got the error:

list indices must be integers, not tuples.

I searched this error, but I found it was actually a very general error and none of the problems that instigated this error were actually for my type of problem.

I suppose I could extract the elements 1 at a time and then merge them, but my gut tells me there is a more "pythonic" way about this.

Also a slightly more complicated form of this problem I ran into was building a new list from an existing list of lists:

new_list = []
for i in range(len(my_list)):
    new_list.append(my_list[i][2,7])

extract the elements one at a time. no need to complicate such a simple and fast operation. — Dimitris Fasarakis Hilliard, Jul 15 '17 at 13:04
@JimFasarakisHilliard True maybe in hindsight that example was too simple. I have included another example of the same problem I ran into. — Arash Howaida, Jul 15 '17 at 13:08
use slicing with steps if I am understanding? `my_list[2,8,5]` — gold_cy, Jul 15 '17 at 13:11
@aws_apprentice That's what I thought I should do too, but I'm puzzled why I got the list indices error. — Arash Howaida, Jul 15 '17 at 13:13

score 5 · Accepted Answer · answered Jul 15 '17 at 13:20

Normally I would just use operator.itemgetter for this:

>>> my_list = list(range(10))
>>> import operator
>>> list(operator.itemgetter(2, 7)(my_list))
[2, 7]

It also allows getting an arbitrary amount of list elements by their indices.

However you can always use NumPy (that's an external package) and it's integer slicing for this (but it won't work for normal Python lists, just for NumPy arrays):

>>> import numpy as np
>>> my_arr = np.array(my_list)
>>> my_arr[[2, 7]]
array([2, 7])

score 1 · Answer 2 · answered Jul 15 '17 at 13:15

1

In [1]: myList = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
In [2]: myList[2:8:5]
Out[2]: [2, 7]

myList[start:end:stride]

Hope this helps.

answered Jul 15 '17 at 13:15

user3053452

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List cherry picking slicing method

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