Summary
The chapter explains zero-based indexing and slicing for sequences, showing how to access individual elements, extract substrings with start, stop, and step parameters (including negative indices for reverse traversal), and highlights that strings are immutable.

General domain of usage
Text processing

Strings are sequences: each character has a position (an **index**). Python uses **zero-based** indexing, so the first character is at index `0`. You can take single characters with **indexing** and ranges of characters with **slicing**.

Watch this video for a clear, visual walkthrough of how indexing and slicing work in Python strings. You'll see real examples of accessing single characters, extracting substrings, using negative indices, and reversing strings—all step by step. This demonstration will help you confidently read and write code that manipulates string data using these essential techniques.

## Indexing

Use square brackets with a single position.


s = "python"
print(s[0])    # 'p'   (first character)
print(s[5])    # 'n'   (sixth character)

s = "python"
print(s[-1])   # 'n'   (last character)
print(s[-2])   # 'o'   (second from the end)

Indexing must hit an existing position, otherwise you get `IndexError`.


s = "python"
print(s[10])  # IndexError: string index out of range

Also, strings are **immutable**, so you can read `s[i]` but not assign to it.

s = "python"
s[0] = 'P'  # TypeError: 'str' object does not support item assignment

## Slicing

A slice uses `start:stop:step` and returns a **new** string. `stop` is **exclusive** (it's not included).


s = "python"
print(s[1:4])    # 'yth'   (indices 1,2,3)
print(s[:4])     # 'pyth'  (start defaults to 0)
print(s[3:])     # 'hon'   (stop defaults to len(s))
print(s[::2])    # 'pto'   (every 2nd character)

Slices are **forgiving**: going past the ends just trims to valid bounds (no error).


### Negative Indices and Reversing

You can mix negative indices in slices, and a negative `step` walks backward.


s = "python"
print(s[-3:])    # 'hon'   (last three)
print(s[::-1])   # 'nohtyp'  (reverse)

`step` cannot be `0`. Leaving out `step` implies `1`. Leaving out `start` or `stop` means "from the beginning" / "to the end".

Note

Which statement **raises an error** for `u = "hello"`?

A focused, beginner-friendly course on working with numbers in Python. Learn about integers, floats, arithmetic operations, type conversion, and how to avoid common pitfalls when handling numerical data.

Indexing and Slicing

Indexing

Slicing

Negative Indices and Reversing

1. What value will this code output?

2. What value will this code output?

3. Which statement raises an error for `u = "hello"`?