Python slice Function |::-1 shorthand, (Array, List, Tuple, String)

Python slice function returns a slice object that can use used to slice strings, lists(Array), tuple, etc. Means slice object is used to specify how to slice a sequence like strings, lists, tuples.

Syntax

slice(start, stop, step)

Parameters

• start  – The slicing of the object starts. Optional and Default is NONE.
• stop – The slicing stops at index stop -1 (last element) and it’s required.
• step – Determines the increment between each index for slicing. Optional and Default is NONE.

Return value:

It returns a slice object.

Example of Python slice Function

Let’s do coding:-

Create a slice object

# contains indices (0, 1, 2)
obj1 = slice(3)
print(obj1)
 
# contains indices (1, 3)
obj2 = slice(1, 5, 2)
print(slice(1, 5, 2))

Output:

slice(None, 3, None)
slice(1, 5, 2)

1. String slicing

Example of python slice string. First create a slice object and then use to it slice string. Passing only stop value.

# String slicing
str = 'Hello World'
s_obj = slice(5)

print(str[s_obj])

Output: Hello

Read more: Python slice string Examples

2. List slicing

Lst = [1, 2, 3, 4, 5]
s_obj = slice(3)

print(Lst[s_obj])

Output: [1, 2, 3]

Read more: Python list slice | Get specific sets of sub-elements Array

3. Tuple slicing

Same as String and list you can slice tuple:-

# Tuple slicing 
Tup = (1, 2, 3, 4, 5)
s_obj = slice(3)

print(Tup[s_obj])

Output: (1, 2, 3)

Python slice shorthand | Understanding slice notation

Slice notation is used to extract a substring.

a[start:stop]  # items start through stop-1
a[start:]      # items start through the rest of the array
a[:stop]       # items from the beginning through stop-1
a[:]           # a copy of the whole array

There is also the step value, which can be used with any of the above:

a[start:stop:step] # start through not past stop, by step

List Slicing Shorthand

li = ['a', 'b', 'Python', 'z', 'Tutorial']
print(li[:3])

print(li[3:])

print(li[:])

Output:

[‘a’, ‘b’, ‘Python’]
[‘z’, ‘Tutorial’]
[‘a’, ‘b’, ‘Python’, ‘z’, ‘Tutorial’]

::-1 python Slice Notation

You can now easily extract the elements of a list that have even indexes:

This also works for List, arrays, and strings:

# Tuple slicing
Tup = (1, 2, 3, 4, 5)
print(Tup[::2])

Output: (1, 3, 5)

Q: How to Python slice list by value

Answer: Use bisect module as part of the standard library. See the below example for slice the list by value in python.

import bisect
 
lst = [1, 3, 5, 6, 8, 9, 11, 13, 17]
for val in range(19):
    pos = bisect.bisect_right(lst, val)
    print(val, '->', lst[max(0, pos - 3):pos])

Output:

Python list slice loop

This code snippet to be very interesting.

a = [0, 1, 2, 3]

for a[-1] in a:
    print(a)

Output:

[0, 1, 2, 0]
[0, 1, 2, 1]
[0, 1, 2, 2]
[0, 1, 2, 2]

Do comment if you have any doubts and suggestion on this tutorial.

Note:
IDE: PyCharm 2020.1.1 (Community Edition)
macOS 10.15.4
Python 3.7
All Python Examples are in Python 3, so Maybe its different from python 2 or upgraded versions.