Python pass slice as argument

Question

I want to pass a slice to a function so I can select a portion of a list. Passing as a string is preferable as I will read the required slice as a command line option from the user.

def select_portion(list_to_slice, slicer):
    return(list_to_slice[slicer])

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
slicer = "1:4"

print(select_portion(numbers, slicer))

I get the following:

TypeError: list indices must be integers or slices, not str

which is understandable but I don't know how to modify this to get the intended output of:

[1, 2, 3]

score 12 · Answer 1 · edited Mar 21 '19 at 13:51

12

Just use slice:

def select_portion(list_to_slice, slicer):
    return(list_to_slice[slicer])

sl = slice(1, 4, None)

select_portion([1,2,3,4,5], sl)

edited Mar 21 '19 at 13:51

han solo

6,390
1
15
19

answered Mar 21 '19 at 13:47

Netwave

40,134
6
50
93

This is nice but doesn't allow you to pass it in as a string? – Sandy Mar 21 '19 at 13:54
@Sandy You could do `>>> slice(*(int(x) for x in '1:4'.split(':')]))` `slice(1, 4, None)`. But i am not sure, if this is frowned upon. If you prefer `map` instead of `list` comprehension `>>> slice(*map(int, x.split(':')))` – han solo Mar 21 '19 at 13:57
@hansolo it's not a great idea... any arguments can be missed... for instance `::3`, `:3:-1` or even just `:` and `::` are valid. – Jon Clements Mar 21 '19 at 15:36
@JonClements Yes, i see the point – han solo Mar 21 '19 at 16:50

score 8 · Answer 2 · answered Mar 21 '19 at 13:47

8

You want the slice constructor. In that case, that would be slice(1, 4, None) (which corresponds to 1:4:).

answered Mar 21 '19 at 13:47

gmds

19,325
4
32
58

Python pass slice as argument

2 Answers2