A piece of text in Python is called a string: a string of characters, one after the other. You have been making strings since your very first print, and joining them together with + since the chapter on variables.

So far a string has always been handled in one piece. It gets stored, printed or joined onto another string, but nothing ever looks inside it. This chapter is about looking inside: how long a string is, what its letters are, and which part of it you want. Taking strings apart and building new ones out of the pieces is called string manipulation.

How long is a string?

len gives you the number of characters in a string, which is called its length.

text_length = len("Dave")
print(text_length)

That prints 4. It is the same len you used to count the items in a list, and it does the same job: a list is counted in items, a string is counted in characters.

Every character counts, not only the letters. A space is a character, and so is a full stop, a digit or an exclamation mark:

print(len("Dave"))
print(len("Ada Lovelace"))
print(len("Hi!"))
print(len(""))

Ada Lovelace has eleven letters and one space, so its length is 12. The last line measures the empty string, two speech marks with nothing between them, and its length is 0.

Using the length in a decision

len gives back a whole number, so it can go anywhere a number can go, including a condition:

username = "dragon"

if len(username) < 3:
    print("That username is too short")
else:
    print("Welcome, " + username)

dragon is six characters long, so the condition is false and the else branch runs. This is how a program checks that a password is long enough, or that a user has typed anything at all.

Changing the case

A letter has two forms, a capital letter and a small one. These are called upper case and lower case, and Python can convert a whole string to either one.

text = "Hello World"

print(text.upper())
print(text.lower())

HELLO WORLD
hello world

upper and lower are written after the string with a full stop in between, the same way append is written after a list. The empty brackets on the end are not optional: without them, Python does not run the conversion.

Only letters have a case. Digits, spaces and punctuation come through a conversion exactly as they went in, so "Room 101!".upper() gives ROOM 101!.

text = "Hello World"
shout = text.upper()

print(shout)
print(text)

Why convert the case at all?

Python compares strings exactly, capital letters included, so "yes", "Yes" and "YES" are three different strings. Users type all three.

Converting the answer before checking it means one comparison covers every way of typing it:

print("Do you want to play again?")
answer = input()

if answer.lower() == "yes":
    print("Here we go!")
else:
    print("Goodbye")

Run it a few times and answer yes, Yes and YES. All three start the game.

Checking the case

Python can also tell you what case a string is already in, without changing it. isupper answers the question “is this string all capitals?” and islower answers “is this string all small letters?”. Each one gives back a Boolean, True or False:

print("HELLO".isupper())
print("hello".islower())
print("Hello".isupper())

The first two print True. The third prints False, because Hello has one capital letter and four small ones, so it is not all capitals. Ask islower the same question and it says False too: a string with a mix of both is neither.

Only the letters are looked at, so "HELLO 123!".isupper() is True. The digits and punctuation have no case and do not count against it.