Slicing & Indexing

Introduction

Slicing with [start:stop:step] works on any sequence: list, tuple, string. Negative indices count from the end.

Understanding the topic

Core concepts to understand:

• xs[1:4] — items 1..3.
• xs[::-1] — reverse.
• xs[::2] — every other.
• Slicing copies; reassignment slices replace in place.

Syntax reference

Visual flow / code:

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

xs[1:4]    # [1, 2, 3]
xs[:3]     # [0, 1, 2]
xs[-2:]    # [4, 5]
xs[::-1]   # reversed
xs[::2]    # [0, 2, 4]

# Replace a slice
xs[1:3] = [10, 20, 30]   # grows the list

# Strings slice the same way
"abcdef"[1:4]   # 'bcd'

Execution workflow

Real-world use

Slicing is one of Python's most loved features — it makes data wrangling code read like English: data[-100:], signal[::2].

Best practices

• Use slicing for copies: new = old[:].
• Step -1 reverses any sequence.

Hands-on exercise

Interview preparation — practice these questions:

• What does xs[::-1] do?
• Slice vs index — what's the difference?
• Slice assignment behavior?

Summary

In summary: [start:stop:step] is universal. Negative indices count from end.