string-formatting.md

Python String Formatting Guide

This guide is meant to teach you various tips and tricks on string formatting in Python! Whether you are preparing a message or displaying output, learning how to format strings is essential in Python. This guide will introduce you to various methods and best practices.

Using Different Quote Marks for Strings

Strings in Python can be defined in several ways, depending on the need for including certain characters within the string or dealing with multi-line strings.

Single (') vs Double (") Quotes

# A simple string defined within single quotes.
s1 = 'Hello, world!'
# A simple string defined within double quotes.
s2 = "Hello, world!"

There is no difference between single and double quotes in Python. You can use either one to define a string. However, you can use one type of quote inside the other to avoid syntax errors.

# A string with single quotes inside double quotes.
s1 = "It's a lovely day!"
# A string with double quotes inside single quotes.
s2 = 'She said: "Hello!"'

Double Quotes (")

Use double quotes when your string contains single quotes (like apostrophes) to avoid syntax errors.

# A string with an apostrophe inside double quotes.
"It's a lovely day!"

Triple Quotes (''') or (""")

Triple single quotes or triple double quotes are used for strings that span multiple lines, which can include line breaks. This is meant to replace the use of \n for line breaks in strings.

# A multi-line string using triple single quotes.
s1 = '''
This string spans
multiple lines
'''
# A multi-line string using triple double quotes.
s2 = """
This string spans
multiple lines
"""
# A string with line breaks using `\n`.
s3 = 'This string\nspans multiple\nlines'
print(s1==s2==s3) # True

The format Method

The format method is used to insert specified values into a string with placeholder braces {}.

Basic Usage of format

Insert values into placeholders in the order they appear.

# A string with placeholders replaced by values passed to `format`.
s1 = '{} {}'.format('Hello', 'World')
print(s1) # 'Hello World'

Positional Formatting with format

Specify the order of placeholders using the position of the arguments in the format method.

# A string where placeholders are filled based on position provided.
s1 = '{1} {0}'.format('World', 'Hello')
print(s1) # 'Hello World'

Keyword Formatting with format

Use keywords in placeholders and provide corresponding values in the format method.

# A string formatted using keyword arguments.
s1 = '{greeting} {noun}'.format(greeting='Hello', noun='World')
print(s1) # 'Hello World'

Formatting Numbers with format

Control the formatting of numbers, like showing a certain number of decimal places.

# Formatting floating-point numbers to show three decimal places.
s1 = 'Pi is approximately {0:.3f}'.format(3.14159)
print(s1) # 'Pi is approximately 3.142'

There are many more ways to format strings using the format method. You can learn more about them in the Python documentation.

f-strings (Formatted String Literals)

F-strings are a more readable and concise way to embed expressions inside string literals using curly braces {}.

Basic f-string

Directly include variables and simple expressions inside string literals.

# Using f-string to include variables directly within the string.
name = 'World'
s1 = f'Hello, {name}!'
print(s1) # 'Hello, World!'

Expression Evaluation in f-strings

Evaluate expressions right inside the string literals.

# An f-string that evaluates an expression within the string.
s1 = f'The sum of 2 and 3 is {2 + 3}.'
print(s1) # 'The sum of 2 and 3 is 5.'

We can use any valid Python expression inside the curly braces of an f-string, but we cannot use multi-line statements.

Formatted Expressions in f-strings

Format numbers or perform operations directly within the curly braces of f-strings.

# Using f-strings to format a floating-point number to three decimal places.
pi = 3.14159
s1 = f'Pi is approximately {pi:.3f}'
print(s1) # 'Pi is approximately 3.142'

Raw Strings

A raw string tells Python to ignore all escape characters (like \n for new line) and print the string exactly as it is.

Using Raw Strings

Useful when dealing with regular expressions or Windows file paths.

# A raw string to maintain actual backslashes in output.
s1 = r'C:\path\to\file'
s2 = "C:\\path\\to\\file"
print(s1==s2) # True

A raw string can be combined with other string formatting methods like f-strings.

file = '1.pdf'
s1 = f'C:\\path\\to\\{file}'
s2 = rf'C:\path\to\{file}'
print(s1==s2) # True

f-strings vs format

In most cases, choosing between f-strings and format is a matter of personal preference. However, there are some cases where one is more suitable than the other.

For example, format is separate from the string literal, so it can be used to format strings that are not known when writing the code. Try running the following code:

user_format = input('Enter format: ')  # Try entering: {}*-*{}
user_greeting = user_format.format('Hello', 'World')
print(user_greeting)  # 'Hello*-*World'

This is also true if you would like to re-use the same format for multiple strings.

user_format = '{1} {0}'
user_greeting = user_format.format('Hello', 'World')
user_farewell = user_format.format('Goodbye', 'World')
print(user_greeting) # 'World Hello'
print(user_farewell) # 'World Goodbye'

On the other hand, f-strings are more readable and concise, so they are preferred when the string is known when writing the code, and there is no need to re-use the format.

name = 'World'
s1 = f'Hello, {name}!'
print(s1) # 'Hello, World!'