String Methods

In the last lesson you learned to build strings and pick out parts of them. Python also gives every string a set of built-in tools for changing case, cleaning up spaces, finding and replacing text, and more. These tools are called methods.

What is a method?

A method is a function that belongs to a value. You call it by writing a dot after the value, then the method name and parentheses:

text = "hello"
print(text.upper())

Read text.upper() as "the upper-case version of text".

Don't forget the parentheses. They're what actually run the method. Without them, Python doesn't call it, and you get a description of the method instead of a result:

text = "hello"
print(text.upper)
print(text.upper())

String methods never change the original string (strings can't be changed, as you saw in the last lesson). They give you back a new string, which you can print or store in a variable:

text = "hello"
shout = text.upper()

print(shout)
print(text)   # still lower case

Changing case

text = "learn Python today"

print(text.upper())
print(text.lower())
print(text.title())
print(text.capitalize())

Notice that capitalize() turned Python into python: it lowers everything except the very first letter. Digits, spaces and punctuation are left as they are by all four methods.

lower() is especially useful for comparing text that people type, because "Yes", "YES" and "yes" all become "yes".

Cleaning up spaces with strip()

strip() removes spaces from both ends of a string, but not from the middle. It's very useful for cleaning up what people type:

messy = "   hello there   "

print("[" + messy + "]")
print("[" + messy.strip() + "]")

The square brackets are just there so you can see where the spaces start and end.

messy = "   hello there   "

print("[" + messy.lstrip() + "]")
print("[" + messy.rstrip() + "]")

Chaining methods

Because each method gives back a new string, you can call another method straight away. Python works from left to right:

name = "   ADA lovelace  "
print(name.strip().title())

First strip() removes the spaces, then title() capitalises each word.

Finding and replacing

sentence = "I like cats. Cats are great."

print(sentence.replace("cats", "dogs"))
print(sentence.count("a"))
print(sentence.startswith("I"))
print(sentence.endswith("!"))

Notice that replace("cats", "dogs") didn't change Cats with a capital C. All of these methods are case-sensitive: "c" and "C" are different characters. A common fix is to lower the text first, as in sentence.lower().count("cats").

find() tells you the index where some text starts, or -1 if it isn't there:

sentence = "I like cats. Cats are great."

print(sentence.find("are"))
print(sentence.find("birds"))

"are" starts at index 18 (counting from 0, spaces included). If the text appears more than once, find() gives the position of the first match. -1 is never a real position, so it safely means "not found".

Checking for text with in

The in operator asks "does this text appear inside that text?" and answers True or False:

email = "[email protected]"

print("@" in email)
print("gmail" in email)

Like the methods above, in is case-sensitive: "Gmail" in email would also be False. You'll use True and False answers like this a lot once you learn if statements.

Use in when you only need a yes/no answer, and find() when you need to know where the text is.

Splitting and joining

split() breaks a string into pieces. It gives you a list, a collection of values in square brackets. Lists get a whole lesson in Part 2; for now, just know that you can pick items from a list by index, exactly like characters in a string:

sentence = "the quick brown fox"
words = sentence.split()

print(words)
print(words[0])
print(words[-1])

With no argument, split() splits on spaces. Give it a character to split somewhere else:

csv_line = "apple,banana,cherry"
print(csv_line.split(","))

The separator itself is removed, and nothing else is. So if the text has a space after each comma, the spaces stay in the pieces:

print("apple, banana, cherry".split(","))
print("apple, banana, cherry".split(", "))

Plain split() with no argument is smarter: it splits on any run of spaces, tabs or new lines, and ignores spaces at the ends. " a b ".split() gives ['a', 'b'].

join() does the opposite: it glues pieces back together with a separator in between:

words = ["the", "quick", "brown", "fox"]
print("-".join(words))
print(" ".join(words))

The syntax looks backwards at first: you call join() on the separator, and pass it the list. Read "-".join(words) as "join the words, with - between them". The separator only goes between items, not at the start or end. Every item in the list must be a string; numbers have to be converted with str() first.

Methods inside f-strings

You can call methods inside the braces of an f-string:

name = "ada"
print(f"Hello, {name.title()}! Your name has {len(name)} letters.")

Exercises

Exercise 1: Clean up a username

Users often type extra spaces and random capitals. Clean up the username so it prints ada_lovelace.

username = "   Ada_LoveLace  "

Exercise 2: Fix the typo

The word Pyton is misspelled twice. Use a method to print the sentence with both fixed: I love Python. Python is fun!

sentence = "I love Pyton. Pyton is fun!"

Exercise 3: Make it loud

Remove the extra spaces, turn the message into capitals and add an exclamation mark, so it prints HELLO WORLD!

message = "   hello world   "

Exercise 4: Email parts

Split the email address into the username and the domain, and print them as Username: ada and Domain: example.com.

email = "[email protected]"

String methods at a glance

A quick reference for later, including some useful methods this lesson didn't cover. Remember: every method gives back a new value and leaves the original string unchanged. Results are shown with quotes, so you can see exactly where each string starts and ends.

Changing case

Cleaning up and replacing

Finding and counting

Checking what's inside

Splitting and joining

Lining up text

Summary

• A method is a function that belongs to a value: text.upper().
• String methods give back a new string. The original never changes.
• upper(), lower() and title() change case. strip() removes spaces from the ends.
• replace() swaps text, count() counts it, and find() gives its position (or -1).
• "x" in text checks whether some text appears in a string.
• split() breaks a string into a list of pieces, and "-".join(pieces) glues them back together.