Strings

Python strings are what we call objects, meaning that they have both state and behavior. In contrast, data types like int, float, and bool only have a single value.

When we say that strings have state and behavior, we are referring to two important properties of strings:

1. state–strings are a certain sequence of characters in a certain order. We can think of this as the value of the string.
2. behavior–strings can do things, like tell us whether or not they are uppercase or count the number of times a certain character occurs in them.

In this section, we’ll look at how to put special characters in strings and accessing specific characters in a string (for instance, the 3rd character). These are all topics associated with the state of a string.

To learn more about string behavior, see the section on using string functions.

Escaping Characters in Strings

Also known as: including special characters in strings.

Some characters won’t work for us to just type into a string. For instance, what if our string includes quotation marks? What if our string spans multiple lines? What if our string needs to include a tab character?

To put these characters into strings, we will need to escape them. This means that we will write them in a special format so that python knows what we want in the string while still being able to type it as a regular string. Our escape character is the backslash \. We will type the backslash before the character that we’d like to escape. For instance, to print ", we’ll type "\"".

Say we’d like to print the string: they said "hello".

# one strategy is to use single quotes around the string
print('they said "hello"')  

# the other strategy is to _escape_ the quotes using a backslash
print("they said \"hello\"") 

Our most important characters that we’ll need to escape are:

• \": a double quote
• \': a single quote
• \n: a newline
• \t: a tab

Challenge: how do you think that you could print a backslash itself?

Take a look at the following print statements and consider what you think they’ll print, then run the code.

print("they didn't say \"goodbye\"")

print('they didn\'t say "goodbye"')

print("super\ncali\nfragilistic\n\nexpialidocious")

print("super\tcali\tfragilistic\n\texpialidocious")

String Indexing

String indexing is how we talk about accessing a specific character in a string. Since strings are made of a sequence of characters in a specific order, we’d like to be able to access the 1st, 2nd, 3rd, etc character of the string.

Let’s take a look at the following string:

Strings are a good example of something that is very common in computer science, which is that they are 0-indexed, meaning that each character has an index (where it occurs in the string) and that the indices start at the number zero.

This string has 8 characters that go from index 0 to index 7.

To access a character at a given index, we use the syntax string_variable[index_number].

Take a look at the following code:

# TODO: run this code.
# Play around with the integer value between the square brackets

s = "dinosaur"
print(s[0])

# TODO: run this code.
# What happens if we give an index value for an index that doesn't
# exist in the string?
# Answer:

s = "dinosaur"
print(s[8])