Think Python, 3rd Edition

Chapter 8. Strings and Regular Expressions

Strings are not like integers, floats, and booleans. A string is a sequence, which means it contains multiple values in a particular order. In this chapter we’ll see how to access the values that make up a string, and we’ll use functions that process strings.

We’ll also use regular expressions, which are a powerful tool for finding patterns in a string and performing operations like search and replace.

As an exercise, you’ll have a chance to apply these tools to a word game called Wordle.

A String Is a Sequence

A string is a sequence of characters. A character can be a letter (in almost any alphabet), a digit, a punctuation mark, or whitespace.

You can select a character from a string with the bracket operator. This example statement selects character number 1 from fruit and assigns it to letter:

fruit = 'banana'
letter = fruit[1]
       

The expression in brackets is an index, so called because it indicates which character in the sequence to select. But the result might not be what you expect:

letter
       

'a'
       

The letter with index 1 is actually the second letter of the string. An index is an offset from the beginning of the string, so the offset of the first letter is 0:

fruit[0]
       

'b'
       

You can think of 'b' as the 0th letter of 'banana'—pronounced “zero-eth.”

The index in brackets can be a variable:

i = 1
fruit[i]
       

'a'
       

Or an expression that contains variables and operators:

fruit[i+1]
       

'n'
       

But the value of the index has to be an integer—otherwise ...