Strings

A string is a piece of text: a name, a sentence, a password, a whole file. Strings are one of the types you'll use most. In this lesson you'll learn how to build strings and pick out parts of them. The next lesson covers the many built-in tools for changing them.

Creating strings

Put text inside single or double quotes. For text that spans several lines, use triple quotes:

first = "Hello"
second = 'World'
poem = """Roses are red,
Violets are blue,
Python is fun,
And so are you."""

print(first, second)
print(poem)

Triple quotes can be """ or '''. The line breaks you type inside them become part of the string, so poem prints on four lines.

A string can also be empty: "" is a string with no characters in it. It's useful as a starting value that you add to later.

Joining and repeating

+ joins strings end to end, and * repeats a string:

first_name = "Ada"
last_name = "Lovelace"

full_name = first_name + " " + last_name
print(full_name)

print("ha" * 3)
print("-" * 20)

Notice the " " in the middle: + doesn't add spaces for you. Without it you'd get AdaLovelace.

Both operators have rules about what they work with:

Counting characters with len()

len() tells you how many characters a string has. Spaces and punctuation count too:

print(len("Python"))
print(len("Hello, World!"))
print(len(""))   # empty string: 0

Indexing: getting one character

Each character in a string has a numbered position called an index. Python starts counting at 0, not 1, so the first character is at index 0. Negative indexes count backwards from the end:

 P   y   t   h   o   n
 0   1   2   3   4   5
-6  -5  -4  -3  -2  -1

Put the index in square brackets after the string:

Predict first: using the diagram above, what will each line print?

word = "Python"

print(word[0])
print(word[3])
print(word[-1])
print(word[-2])

word[0] is always the first character and word[-1] is always the last, however long the string is.

Because counting starts at 0, the last index is always one less than the length. "Python" has 6 characters, so its indexes run from 0 to 5. Asking for a position that doesn't exist raises an IndexError:

word = "Python"
print(word[10])

Slicing: getting part of a string

A slice gives you a piece of a string. word[start:end] takes the characters from start up to, but not including, end:

word = "Python"

print(word[0:2])   # positions 0 and 1
print(word[2:5])   # positions 2, 3 and 4

end is left out for the same reason as with range() later on: it makes the arithmetic easy. end - start is the number of characters you get, so word[2:5] gives 5 - 2 = 3 characters: tho.

Leave out start to begin at the start, or end to go all the way to the end:

word = "Python"

print(word[:3])    # the first 3 characters
print(word[3:])    # from position 3 on
print(word[-3:])   # the last 3 characters

Here are the patterns you'll use most, for word = "Python":

Unlike indexing, slicing never raises an IndexError. If the numbers go past the end, you just get whatever characters exist, or an empty string:

word = "Python"

print(word[2:100])
print(word[10:20])
print(len(word[10:20]))

Reversing a string

One more pattern is worth memorising: word[::-1] gives the string reversed.

print("Python"[::-1])
print("stressed"[::-1])

word = "Python"

# every second character
print(word[::2])

# every second, from position 1
print(word[1::2])

# backwards
print(word[::-1])

# backwards, every second one
print(word[::-2])

Strings can't be changed

Once a string is created, you can't change a single character inside it:

word = "Python"
word[0] = "J"

That's a TypeError: Python won't let you assign to a position inside a string.

Instead, build a new string from the pieces you want:

word = "Python"
new_word = "J" + word[1:]
print(new_word)

Programmers say strings are immutable, which just means "can't be changed in place".

Special characters

A backslash \ inside a string starts a special character. You met these in the first lesson; here they are again for reference. Each one counts as one character, so len("a\nb") is 3:

print("Line one\nLine two")
print("Name:\tAda")
print("She said \"hi\"")
print("C:\\Users\\Ada")

print("C:\new_folder\notes.txt")

print(r"C:\new_folder\notes.txt")

Exercises

Exercise 1: Initials

Print the initials of the name below, followed by full stops: A.L.

first_name = "Ada"
last_name = "Lovelace"

Exercise 2: The middle part

Use a slice to print just yth from the word.

word = "Python"

Exercise 3: Underline a title

Print the title, and underneath it a line of = signs exactly as long as the title:

Shopping list
=============

title = "Shopping list"

Exercise 4: Palindrome check

A palindrome reads the same forwards and backwards. Print True if word is a palindrome.

word = "racecar"

String operations at a glance

A quick reference for later. Each example starts from s = "Python". Results are shown with quotes, so you can see exactly where each string starts and ends.

Strings also have dozens of built-in methods, such as upper() and replace(). They're the topic of the next lesson, String Methods.

Summary

• Strings are text in quotes. Use triple quotes for text over several lines.
• + joins strings, * repeats them, and len() counts characters.
• Indexes start at 0: s[0] is the first character and s[-1] the last.
• s[start:end] slices out a piece. The end is not included.
• s[:3] gives the first 3 characters, s[-3:] the last 3, and s[::-1] reverses.
• Strings can't be changed in place. Build a new string instead.
• \n starts a new line, and \t adds a tab.