What is Slicing?

While indexing gives you a single element, slicing gives you a range of elements. Think of it like cutting a section from a loaf of bread -- you specify where to start cutting and where to stop.

Basic Slice Syntax

numbers = [10, 20, 30, 40, 50, 60, 70]
#           0    1    2    3    4    5    6

# Get elements from index 1 to 4 (not including 4)
subset = numbers[1:4]
print(subset)

# Get elements from index 2 to 6
print(numbers[2:6])

Omitting Start, Stop, or Step

You can omit any of the three values, and Python will use defaults:

• Omit start: defaults to the beginning (index 0)
• Omit stop: defaults to the end of the list
• Omit step: defaults to 1

numbers = [10, 20, 30, 40, 50, 60, 70]

# From beginning to index 3
print(numbers[:3])

# From index 4 to the end
print(numbers[4:])

# Get the entire list (copy)
print(numbers[:])

# First three elements
print(numbers[:3])

# Last three elements
print(numbers[-3:])

Using the Step Parameter

The third value in the slice specifies the step -- how many elements to skip between each selected element.

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

# Every other element
print(numbers[::2])

# Every third element
print(numbers[::3])

# Every other element from index 1 to 8
print(numbers[1:8:2])

students = ["Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"]

# Students at even indexes (0, 2, 4)
team_a = students[::2]
print("Team A:", team_a)

# Students at odd indexes (1, 3, 5)
team_b = students[1::2]
print("Team B:", team_b)

Reversing with Slicing

Using a step of -1 reverses the list. This creates a new reversed list without modifying the original.

numbers = [1, 2, 3, 4, 5]

# Reverse the list
reversed_nums = numbers[::-1]
print("Reversed:", reversed_nums)
print("Original:", numbers)   # Original unchanged

Negative Step with Start and Stop

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

# From index 7 down to index 2 (not including 2)
print(numbers[7:2:-1])

# Every other element in reverse
print(numbers[::-2])

Copying Lists Safely

# This does NOT copy the list!
original = [1, 2, 3]
not_a_copy = original    # Both point to same list
not_a_copy.append(4)
print("Original:", original)    # Original is also changed!
print("Not a copy:", not_a_copy)

# Use slicing to create a true copy
original = [1, 2, 3]
copy1 = original[:]       # Slice copy
copy2 = original.copy()   # .copy() method

copy1.append(99)
print("Original:", original)     # Unchanged!
print("Copy 1:", copy1)
print("Copy 2:", copy2)

Slicing with Negative Indexes

You can mix positive and negative indexes in slices for flexible data extraction.

letters = ["a", "b", "c", "d", "e", "f", "g"]

# Everything except the first and last
print(letters[1:-1])

# Last four elements
print(letters[-4:])

# Everything except the last two
print(letters[:-2])

short = [1, 2, 3]
print(short[0:100])   # No error, returns all elements
print(short[10:20])   # No error, returns empty list