Strings, Part I

Strings, Part I#

Note

Source: Drawn from the SE4ML Python chapter (chapter_python.rst, lines 2312–2355) and the C# edition (data/strings1.rst). String literals, concatenation, repetition, and immutability follow the SE4ML presentation; str() conversion is adapted from the C# edition.

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:

>>> 'Hello'
'Hello'
>>> "Hello"
'Hello'

They are equivalent. Using double quotes inside single-quoted strings (and vice versa) avoids backslashes:

>>> 'She said, "Hello."'
'She said, "Hello."'
>>> "It's a fine day."
"It's a fine day."

For strings spanning multiple lines, use triple quotes — either """ or ''':

>>> poem = """Roses are red,
... Violets are blue."""
>>> print(poem)
Roses are red,
Violets are blue.

String Operations#

The + operator concatenates (joins) two strings:

>>> "Hello, " + "world!"
'Hello, world!'
>>> first = "Ada"
>>> last = "Lovelace"
>>> full = first + " " + last
>>> full
'Ada Lovelace'

The * operator repeats a string:

>>> "ha" * 3
'hahaha'
>>> "-" * 20
'--------------------'

String Length#

len() returns the number of characters in a string:

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

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:

>>> s = "Hello"
>>> s[0] = "J"       # This will cause an error
TypeError: 'str' object does not support item assignment

To “change” a string, create a new one:

>>> s = "J" + s[1:]
>>> s
'Jello'

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

Converting to String#

Use str() to convert other types to strings:

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

Note that you cannot concatenate a string and a number directly:

>>> "The answer is " + 42
TypeError: can only concatenate str (not "int") to str

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