Strings, Part I

A string is a sequence of characters. Strings are one of the most commonly used types in Python.

String Literals

You can write a string literal using either single quotes or double quotes. They are equivalent. Using double quotes inside single-quoted strings (and vice versa) avoids backslashes, and triple quotes — either """ or ''' — let a string span multiple lines. Try editing these:

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>>> 'Hello'
'Hello'
>>> "Hello"
'Hello'
>>> 'She said, "Hello."'
'She said, "Hello."'
>>> "It's a fine day."
"It's a fine day."
>>> poem = """Roses are red,
... Violets are blue."""
>>> print(poem)
Roses are red,
Violets are blue.

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String Operations

The + operator concatenates (joins) two strings, and the * operator repeats a string. Try changing the words and counts below:

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>>> "Hello, " + "world!"
'Hello, world!'
>>> first = "Ada"
>>> last = "Lovelace"
>>> full = first + " " + last
>>> full
'Ada Lovelace'
>>> "ha" * 3
'hahaha'
>>> "-" * 20
'--------------------'

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String Length

len() returns the number of characters in a string. Try it with your own words:

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>>> len("Hello")
5
>>> len("")
0
>>> len("Ada Lovelace")
12

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Strings are Immutable

Strings cannot be changed after they are created. You can create a new string based on an old one, but you cannot modify the original. Trying to assign to a single character raises an error; instead, build a new string. Try it:

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>>> s = "Hello"
>>> s[0] = "J"       # This will cause an error
Traceback (most recent call last):
    ...
TypeError: 'str' object does not support item assignment
>>> s = "J" + s[1:]
>>> s
'Jello'

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Slicing and indexing are covered in the Basic String Operations chapter.

Converting to String

Use str() to convert other types to strings. Try it:

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>>> str(42)
'42'
>>> str(3.14)
'3.14'
>>> "The answer is " + str(42)
'The answer is 42'

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Note that you cannot concatenate a string and a number directly — doing so raises a TypeError:

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>>> "The answer is " + 42
Traceback (most recent call last):
    ...
TypeError: can only concatenate str (not "int") to str

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You must convert first with str(), or use an f-string (see F-Strings and String Formatting).