Python Slicing

The start:stop:step Syntax

A slice takes up to three parts separated by colons: start (inclusive, default 0), stop (exclusive, default end of sequence), and step (default 1). Any part can be omitted.

nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

print(nums[2:5])   # [2, 3, 4]
print(nums[:4])    # [0, 1, 2, 3]
print(nums[6:])    # [6, 7, 8, 9]
print(nums[::2])   # [0, 2, 4, 6, 8]

Negative Indices

Negative numbers count from the end of the sequence, with -1 as the last item. They work in any part of the slice, which makes it easy to grab the last few items without knowing the length.

nums = [0, 1, 2, 3, 4, 5]

print(nums[-3:])   # [3, 4, 5] -> last 3 items
print(nums[:-2])   # [0, 1, 2, 3] -> all but the last 2

Reversing a Sequence

A step of -1 walks backward through the sequence, making [::-1] the shortest way to reverse a list, string, or tuple without a loop.

text = "python"
print(text[::-1])   # 'nohtyp'

nums = [1, 2, 3, 4]
print(nums[::-1])   # [4, 3, 2, 1]

Slicing Different Sequence Types

Slicing works the same way on any Python sequence type. A slice of a list returns a new list, a slice of a string returns a new string, and a slice of a tuple returns a new tuple.