CODEWITHSUNDEEP
python
Zulakis
Zulakis

Reputation: 8384

Python list slicing with string argument

It's possible to slice a python list like this:

>>> list=['a', 'b']
>>> list[0:1]
['a']

However, when passing the index as a string, an error is thrown:

>>> index="0:1"
>>> list[index]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list indices must be integers, not str

How can I specify a list index as a string? What data type is 0:1 in list[0:1], really?

Upvotes: 0

Views: 1182

Answers (4)

seunmi
seunmi

Reputation: 1

Use the [ : ] where you specify the start point and end point ,so you can easily trace what you intend to slice

Upvotes: 0

tobias_k
tobias_k

Reputation: 82899

n:m is syntactic sugar for a slice. You could split your index string, convert its parts to integers, and create a slice from those.

>>> lst = list(range(10))
>>> index = "1:4"
>>> s = slice(*map(int, index.split(':')))
>>> lst[s]
[1, 2, 3]

Works just the same with three parts:

>>> index = "1:9:2"
>>> s = slice(*map(int, index.split(':')))
>>> lst[s]
[1, 3, 5, 7]

If you want to allow for "blank" parts, the conversion gets a little bit more involved:

>>> index = "::-1"
>>> s = slice(*[int(x) if x else None for x in index.split(':')])
>>> lst[s]
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

Upvotes: 7

alec_djinn
alec_djinn

Reputation: 10789

You can use exec() for this.

l = ['a', 'b']
index="0:1"
exec('print(l[{}])'.format(index))

The print is needed only to see the output. You can assign it to a variable and call the variable afterwards instead.

Upvotes: 1

Sнаđошƒаӽ
Sнаđошƒаӽ

Reputation: 17582

Why not just convert your slicing string into integers by splitting on :, and then using them as slicing indices.

list=['a', 'b']
slicer_str = '0:1'
slicer_int = [int(i) for i in slicer_str.split(':')]
print(list[slicer_int[0]:slicer_int[1]])

Upvotes: 1

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