String Methods and Formatting

Python strings are full of handy tools called methods. Because strings are objects, they carry these built-in functions with them.

Case Conversion

You can easily change the casing of a string using methods like upper() and lower().

msg = 'Hello World'
print(msg.upper())

HELLO WORLD

msg = 'HELLO'
print(msg.lower())

hello

Immutability: Predict the Output!

Here is the most important rule about strings: Strings are immutable. That means they cannot be changed in place. When you run a method like upper(), it returns a brand new string.

msg = 'hello'
msg.upper()
print(msg)

hello

Notice that printing msg still outputs hello in lowercase! If you want to keep the uppercase version, you must assign it to a variable: msg = msg.upper().

Cleaning Up Text

The strip() method is perfect for cleaning up messy user input. It removes leading and trailing characters (spaces by default).

msg = '   hello   '
print(f'*{msg.strip()}*')

*hello*

Edge Cases

• strip() removes any combination of the specified characters, not just the exact sequence.

Replacing Substrings

To swap out parts of a string, use replace(). By default, it replaces all occurrences.

msg = 'I like apples'
print(msg.replace('apples', 'bananas'))

I like bananas

Splitting Text into Lists

The split() method breaks a single string into a list of smaller strings. By default, it splits on whitespace, but you can specify any delimiter.

csv = 'one,two,three'
print(csv.split(','))

['one', 'two', 'three']

Edge Cases

• split() with no arguments correctly handles multiple consecutive spaces as a single separator, unlike split(' ').