Python Modify Strings

Real-world scenario

Think about messaging apps. When people send texts, they sometimes type in ALL CAPS, leave extra spaces, or use words that need to be cleaned before display. Apps like WhatsApp or Messenger process these messages so they look neat and professional.

In this lesson, we’ll build a chat message cleaner that uses Python string methods to fix messy text.

Here’s how a messy message might look before and after cleaning:

1
2
Before:   "   HELLO Friend!!   "  
After:    "hello friend!!"

You’ve been hired to create this message cleaner. Let’s learn how Python string methods make this easy.

The core idea

Strings in Python come with built-in methods that let you modify text. Some of the most useful are:

1. .upper() → changes all letters to uppercase.
   Example: "hello".upper() → "HELLO"
2. .lower() → changes all letters to lowercase.
   Example: "HELLO".lower() → "hello"
3. .strip() → removes spaces from the start and end.
   Example: " hi ".strip() → "hi"
4. .replace(old, new) → replaces one word with another.
   Example: "hi there".replace("hi", "hello") → "hello there"
5. .split() → breaks a string into a list of words.
   Example: "apple,banana,grape".split(",") → ["apple", "banana", "grape"]

Why this matters

Messy user input can cause problems. If your program expects "yes" but the user types " YES " in caps with spaces, the program won’t recognize it unless you clean it. String modification ensures consistency and avoids errors.

✨ Quick facts

• .upper() → all uppercase
• .lower() → all lowercase
• .strip() → trims spaces
• .replace() → swaps words
• .split() → breaks into parts

Try it in code

Let’s clean up a message step by step:

1
2
3
4
5
6
message = "   HELLO Friend!!   "

print("Original:", message)
print("Lowercase:", message.lower())
print("Trimmed:", message.strip())
print("Replaced:", message.replace("Friend", "Buddy"))

Output:

1
2
3
4
Original:    HELLO Friend!!   
Lowercase:   hello friend!!   
Trimmed: HELLO Friend!!  
Replaced:    HELLO Buddy!!   

Learn with AI

If you’re using an AI assistant, try this:

Prompt:
Write a Python program that takes a messy chat message, and uses .strip(), .lower(), and .replace() to clean it up and print the result.

The AI will generate neat code you can study and test.

Practice challenge

Excellent progress! Now let’s try a challenge.

Task

You are building a form processor. The input string is:

1
"   WELCOME To PYTHON   "

Your program should:

• Remove extra spaces.
• Convert it all to lowercase.
• Replace "python" with "programming".
• Split the final string into words.

Expected output:

1
['welcome', 'to', 'programming']

Solution

1
2
3
4
5
6
7
8
text = "   WELCOME To PYTHON   "

cleaned = text.strip()          # remove spaces
cleaned = cleaned.lower()       # convert to lowercase
cleaned = cleaned.replace("python", "programming")  # replace word
words = cleaned.split()         # split into list

print(words)

Perfect! You’ve now mastered Python’s string modification methods.