Indexing & Slicing

Because strings are ordered collections of characters, every character in a Python string sits at a specific numbered position called an index. You can grab a single character using brackets [] or extract a section of text using slicing [:].

Index Positions (Start at 0)

Python counts positions starting from 0 from left to right. You can also count backward from -1 starting from the right end:

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Visual Guide: Accessing individual characters using positive and negative indexing.

word = "PYTHON"

# Grab the first character
print(word[0])   # Output: P

# Grab the second character
print(word[1])   # Output: Y

# Grab the last character using negative indexing
print(word[-1])  # Output: N

Slicing Text ([start:stop])

To extract a substring (a smaller piece of the text), use a colon : inside the brackets. Python starts taking characters at start and stops just before reaching stop:

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Visual Guide: Extracting a part of a string using slicing.

language = "PYTHON"

# Grab characters from index 0 up to index 3 (gets index 0, 1, 2)
print(language[0:3])  # Output: PYT

# Leave out the start index to begin from the very first letter
print(language[:4])   # Output: PYTH

# Leave out the stop index to go all the way to the end
print(language[2:])   # Output: THON

Step Slicing ([start:stop:step])

You can add a third number called the step to jump over characters or change the direction of reading:

• A positive step (e.g. 2) moves forward from left to right, skipping numbers.
• A negative step (e.g. -1) moves backwards through the string from right to left, which allows you to reverse text easily!

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Visual Guide: Understanding step slicing to skip characters and reverse strings.

alphabet = "ABCDEFGH"

# Jump every 2 letters
print(alphabet[0:8:2])  # Output: ACEG

# Reverse the entire string with step -1 (moves backwards from right to left)
print(alphabet[::-1])   # Output: HGFEDCBA

Strings Cannot Be Changed (Immutability)

Once a string is created, you cannot swap or replace an individual letter inside it using index numbers. If you need modified text, you create a new string variable:

text = "Cat"
# text[0] = "B"  # TypeError! Python strings cannot be modified in place.

# Right way: create a new string
new_text = "B" + text[1:]
print(new_text)  # Output: Bat

Common Beginner Mistakes

• Zero-Based Indexing: The first character is always at index 0, not 1 (word[0] is "P" in "PYTHON").
• Stop Index Exclusion: In slicing [start:stop], the stop position is excluded (e.g., word[0:3] gives indices 0, 1, 2).
• Index Out of Range: Accessing an index >= len(string) raises an IndexError.

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Quick Summary

• Indexing (word[i]): Access individual characters using positive (left-to-right from 0) or negative (right-to-left from -1) indices.
• Slicing (word[start:stop:step]): Extracts a range of characters up to (excluding) the stop position.
• Shorthands: word[:3] starts from the beginning; word[3:] goes until the end.
• String Reversal: word[::-1] reverses the entire string using a negative step.

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What's Next?

Now that we know how to access characters and slices, can we modify a character in place? Let's uncover one of Python's most important concepts in Mutable vs. Immutable (String Immutability)!

Mutable vs. Immutable

Understand object mutability, memory addresses (id), and why strings cannot be changed in place