Multiline Strings

>>> print(
... """Dear Alice,
...
... Eve's cat has been arrested for catnapping,
... cat burglary, and extortion.
...
... Sincerely,
... Bob"""
... )

# Dear Alice,

# Eve's cat has been arrested for catnapping,
# cat burglary, and extortion.

# Sincerely,
# Bob

Indexing and Slicing strings

​​​​H   e   l   l   o       w   o   r   l   d    !
​​​​0   1   2   3   4   5   6   7   8   9   10   11

Indexing

>>> spam = 'Hello world!'

>>> spam[0]
# 'H'

>>> spam[4]
# 'o'

>>> spam[-1]
# '!'

Slicing

>>> spam = 'Hello world!'

>>> spam[0:5]
# 'Hello'

>>> spam[:5]
# 'Hello'

>>> spam[6:]
# 'world!'

>>> spam[6:-1]
# 'world'

>>> spam[:-1]
# 'Hello world'

>>> spam[::-1]
# '!dlrow olleH'

>>> fizz = spam[0:5]
>>> fizz
# 'Hello'

The in and not in operators

>>> 'Hello' in 'Hello World'
# True

>>> 'Hello' in 'Hello'
# True

>>> 'HELLO' in 'Hello World'
# False

>>> '' in 'spam'
# True

>>> 'cats' not in 'cats and dogs'
# False

Escape characters

An escape character is created by typing a backslash \ followed by the character you want to insert.

Escape character	Prints as
\'	Single quote
\"	Double quote
\t	Tab
\n	Newline (line break)
\\	Backslash

>>> print("Hello there!\nHow are you?\nI\'m doing fine.")
# Hello there!
# How are you?
# I'm doing fine.

Raw strings

A raw string entirely ignores all escape characters and prints any backslash that appears in the string.

>>> print(r"Hello there!\nHow are you?\nI\'m doing fine.")
# Hello there!\nHow are you?\nI\'m doing fine.

% operator

>>> name = 'Pete'
>>> 'Hello %s' % name
# "Hello Pete"

We can use the %d format specifier to convert an int value to a string:

>>> num = 5
>>> 'I have %d apples' % num
# "I have 5 apples"

Formatted String Literals or f-Strings

If your are using Python 3.6+, string f-Strings are the recommended way to format strings.

Python 3 documentation

>>> name = 'Elizabeth'
>>> f'Hello {name}!'
# 'Hello Elizabeth!'

It is even possible to do inline arithmetic with it:

>>> a = 5
>>> b = 10
>>> f'Five plus ten is {a + b} and not {2 * (a + b)}.'
# 'Five plus ten is 15 and not 30.'

Multiline f-Strings

>>> name = 'Robert'
>>> messages = 12
>>> (
... f'Hi, {name}. '
... f'You have {messages} unread messages'
... )
# 'Hi, Robert. You have 12 unread messages'

Formatting Digits

Rounding

>>> a = 3.1415926
>>> f"{a:.2f}"
# '3.14'

Showing as Percentage

>>> a = 0.816562
>>> f"{a:.2%}"
# '81.66%'

Number formatting table

Number	Format	Output	description
3.1415926	{:.2f}	3.14	Format float 2 decimal places
3.1415926	{:+.2f}	+3.14	Format float 2 decimal places with sign
-1	{:+.2f}	-1.00	Format float 2 decimal places with sign
2.71828	{:.0f}	3	Format float with no decimal places
4	{:0>2d}	04	Pad number with zeros (left padding, width 2)
11	{:11d}	11	Right-aligned (default, width 10)
11	{:<11d}	11	Left-aligned (width 10)
11	{:^11d}	11	Center aligned (width 10)

upper(), lower() and title()

Transforms a string to upper, lower and title case:

>>> greet = 'Hello world!'
>>> greet.upper()
# 'HELLO WORLD!'

>>> greet.lower()
# 'hello world!'

>>> greet.title()
# 'Hello World!'

isupper() and islower() methods

Returns True or False after evaluating if a string is in upper or lower case:

>>> spam = 'Hello world!'
>>> spam.islower()
# False

>>> spam.isupper()
# False

>>> 'HELLO'.isupper()
# True

>>> 'abc12345'.islower()
# True

>>> '12345'.islower()
# False

>>> '12345'.isupper()
# False

The isX() string methods

Method	Description
isalpha()	returns True if the string consists only of letters.
isalnum()	returns True if the string consists only of letters and numbers.
isdecimal()	returns True if the string consists only of numbers.
isspace()	returns True if the string consists only of spaces, tabs, and new-lines.
istitle()	returns True if the string consists only of words that begin with an uppercase letter followed by only lowercase characters.

startswith() and endswith() (Optional)

>>> 'Hello world!'.startswith('Hello')
# True

>>> 'Hello world!'.endswith('world!')
# True

>>> 'abc123'.startswith('abcdef')
# False

>>> 'abc123'.endswith('12')
# False

>>> 'Hello world!'.startswith('Hello world!')
# True

>>> 'Hello world!'.endswith('Hello world!')
# True

replace()

The replace() function is like a “search and replace” operation in a word processor

>>> greet = 'Hello Bob'
>>> nstr = greet.replace('Bob','Jane')
>>> print(nstr)
# Hello Jane

join() and split()

join()

The join() method takes all the items in an iterable, like a list, dictionary, tuple or set, and joins them into a string. You can also specify a separator.

>>> ''.join(['My', 'name', 'is', 'Simon'])
'MynameisSimon'

>>> ', '.join(['cats', 'rats', 'bats'])
# 'cats, rats, bats'

>>> ' '.join(['My', 'name', 'is', 'Simon'])
# 'My name is Simon'

>>> 'ABC'.join(['My', 'name', 'is', 'Simon'])
# 'MyABCnameABCisABCSimon'

split()

The split() method splits a string into a list. By default, it will use whitespace to separate the items, but you can also set another character of choice:

>>> 'My name is Simon'.split()
# ['My', 'name', 'is', 'Simon']

>>> 'MyABCnameABCisABCSimon'.split('ABC')
# ['My', 'name', 'is', 'Simon']

>>> 'My name is Simon'.split('m')
# ['My na', 'e is Si', 'on']

>>> ' My  name is  Simon'.split()
# ['My', 'name', 'is', 'Simon']

>>> ' My  name is  Simon'.split(' ')
# ['', 'My', '', 'name', 'is', '', 'Simon']

Removing whitespace with strip(), rstrip(), and lstrip()

>>> spam = '    Hello World     '
>>> spam.strip()
# 'Hello World'

>>> spam.lstrip()
# 'Hello World     '

>>> spam.rstrip()
# '    Hello World'

Keywords

• string (字串): A sequence of characters used to represent text data.
• escape character (跳脫字元): A backslash (\) used to introduce special character sequences in a string, such as \n for newline.
• raw string (原始字串): A string prefixed with r that treats backslashes as literal characters rather than escape sequences.
• string interpolation (字串插值): The process of inserting variable values or expressions into a string placeholder.
• format string (格式化字串): A string containing placeholders that can be replaced with variable values, often using the format() method.
• format specifiers (格式說明子): Special syntax within a format string that defines how a value should be formatted (e.g., .2f for two decimal places).
• f-string (f字串): A string literal prefixed with f that allows inline expression evaluation and formatting directly within the string.