A gentle introduction to f-strings

Published

November 6, 2025

Formatted String Literals, called f-strings for short, are a way for you to include expressions and format values inside of a Python string. F-strings are available in Python 3.6+ (earlier versions use format()).

To create an f-string, put an f in front of your string literal, then use {} curly braces to add variables or expressions. Here’s how you create a basic f-string:

>>> x = 100
>>> century = f"A century is {x} years"
>>> print(century)
'A century is 100 years'

You can see the variable x is embedded directly into the string, producing 'A century is 100 years'.

Using Expressions Inside F-Strings

Here’s where f-strings get really powerful: you can put any Python expression inside the curly braces, not just variables. This means you can do calculations, call functions, or use methods directly inside your string.

Here’s an example with arithmetic:

>>> slices_per_pizza = 8
>>> pizzas = 3
>>> total_pizza = f"Total pizza slices: {slices_per_pizza * pizzas}"
>>> print(total_pizza)
'Total pizza slices: 24'

The expression slices_per_pizza * pizzas is evaluated when the f-string is created, giving you 'Total pizza slices: 24'. You can also call string methods to transform text on the fly:

>>> magic_spell = "abracadabra"
>>> wizard = f"The wizard cast {magic_spell.upper()}!"
>>> print(wizard)
'The wizard cast ABRACADABRA!'

This converts the spell’s name to uppercase, producing 'The wizard cast ABRACADABRA!'. ## Formatting Values

One of f-strings’ most useful features is the ability to control how values are displayed. You add formatting instructions by adding a colon : after the variable name inside the curly braces, giving you precise control over your output.

You control decimal places with .Nf where N is the number of digits. This is particularly important when working with financial data or scientific calculations:

>>> import math
>>> pi_digits = f'The first 10 digits of pi are {math.pi:.10f}'
>>> print(pi_digits)
'The first 10 digits of pi are 3.1415926536'

This limits pi to 10 decimal places: 'The first 10 digits of pi are 3.1415926536'.

Date Formatting

When working with datetime objects, you can use format codes after the colon. Common codes include %B for full month name, %d for day, and %Y for full year:

>>> from datetime import datetime
>>> square_date = datetime(2025, 9, 16)
>>> formatted_date = f"Date: {square_date:%B %d, %Y}"
>>> print(formatted_date)
'Date: September 16, 2025'

This produces a nicely formatted date: 'Date: September 16, 2025'.

Special Conversions

Use !r to see the string representation of a value. This shows quotes around strings and escape characters, which is helpful for debugging:

>>> name = "Marco\nPolo"
>>> f"Repr: {name!r}"
"Repr: 'Marco\\nPolo'"

Here’s what you get: "Repr: 'Marco\\nPolo'". The newline character is displayed as \\n instead of being interpreted. The !a conversion works like !r but uses ascii() to escape non-ASCII characters.

You can also use = after your expression to show both the expression and its value:

>>> movie_rating = 8.7
>>> f"The value of {movie_rating=}"
'The value of movie_rating=8.7'

Where the output This saves you from typing the variable name twice.

F-strings are the modern, preferred way to format strings in Python. They make your code more readable and string output more polished.