Python slice() Function

The slice() function in Python creates a slice object that can be used to extract parts of a sequence like a list, tuple, or string. This is very useful when you want to get a sub-part of a sequence using custom start, stop, and step values.

Syntax

slice(stop)
slice(start, stop[, step])

Parameters:

• start – (Optional) The starting index of the slice. Default is 0.
• stop – The ending index (exclusive).
• step – (Optional) The step size or interval. Default is 1.

Returns:

• A slice object that can be used to extract portions from a sequence.

Example 1: Basic Usage with a List

numbers = [10, 20, 30, 40, 50, 60]
s = slice(1, 4)
print(numbers[s])

[20, 30, 40]

Example 2: Using Step with slice()

letters = ['a', 'b', 'c', 'd', 'e', 'f']
s = slice(1, 6, 2)
print(letters[s])

['b', 'd', 'f']

Example 3: slice() with Strings

text = "HelloWorld"
s = slice(0, 5)
print(text[s])

Hello

How is slice() Different from [start:stop:step]?

Python allows slicing using both slice() and the shorthand notation [start:stop:step]. They do the same thing but slice() is more flexible when you want to pass slicing as a parameter to functions or reuse slicing logic.

Use Case: Reusable Slice Logic

def get_middle_part(seq):
    s = slice(1, -1)
    return seq[s]

print(get_middle_part("Python"))
print(get_middle_part([10, 20, 30, 40]))

ytho
[20, 30]

Common Mistakes

• Forgetting that the stop index is exclusive.
• Passing strings instead of integers to slice().
• Assuming slice() extracts the values itself — it only creates a slice object.

Interview Tip

Use slice() when writing reusable logic or custom sequence functions. Knowing how to slice efficiently is helpful in coding rounds involving arrays or strings.

Summary

• slice() returns a slice object for use with sequences.
• You can specify start, stop, and step.
• Works with lists, tuples, strings, and other slicable types.

Practice Problem

Write a function that takes a list and returns every third element starting from index 0 using slice().

def every_third_element(lst):
    s = slice(0, len(lst), 3)
    return lst[s]

print(every_third_element([1, 2, 3, 4, 5, 6, 7, 8, 9]))

[1, 4, 7]