Python F-Strings | Docs With Examples

Python f-strings (formatted string literals) provide an elegant and efficient way to format strings in your Python programs.

Introduced in Python 3.6, f-strings make string interpolation easier, more readable, and faster than older Python methods like % formatting and .format().

Basic Syntax

F-strings are defined by prefixing a string with f or F, allowing variables and expressions to be embedded directly within curly braces {}, which serve as placeholders.

Take a look at the syntax, it's quite simple and very intuitive:

name = "Alice"
age = 25
print(f"My name is {name} and I am {age} years old.")

Explanation: This f-string directly injects the name and age variables into the string, producing a well-formatted output.

Output:

My name is Alice and I am 25 years old.

Using Expressions in F-Strings

You can include expressions inside f-strings, making them highly versatile. You just need to use your placeholders to include your expressions in the same way you would with a variable:

x = 5
y = 3
print(f"The sum of {x} and {y} is {x + y}.")

Explanation: The expression {x + y} is evaluated within the string, dynamically inserting the result.

Output:

The sum of 5 and 3 is 8.

Formatting Numbers

One of the trickier aspects of using f-strings is formatting numbers, but thankfully you'll see that this is fairly straightforward.

1. Rounding Decimals

pi = 3.14159
print(f"Pi rounded to 2 decimal places: {pi:.2f}")

Explanation: The :.2f format specifier ensures that the number is rounded to two decimal places.

Output:

Pi rounded to 2 decimal places: 3.14

2. Displaying Large Numbers with Commas

salary = 1000000
print(f"Annual salary: ${salary:,}")

Explanation: The :, format specifier automatically inserts commas into large numbers for better readability.

Output:

Annual salary: $1,000,000

3. Displaying Percentages

progress = 0.85
print(f"Task completion: {progress:.0%}")

Explanation: The :.0% format converts the decimal value into a percentage format without decimal places.

Output:

Task completion: 85%

Working with Dates and Time

You can format dates and times directly inside f-strings:

from datetime import datetime

today = datetime.now()
print(f"Today's date: {today:%Y-%m-%d}")

Explanation: The %Y-%m-%d format string extracts and formats the year, month, and day from the datetime object.

Output:

Today's date: 2023-07-15  # Example output, depends on the current date

Multiline F-Strings

Use triple quotes for multi-line f-strings:

name = "Alice"
age = 25
bio = f"""
Name: {name}
Age: {age}
Status: Active
"""
print(bio)

Explanation: Triple quotes allow f-strings to span multiple lines while keeping them easy to read and format.

Output:

Name: Alice
Age: 25
Status: Active

Using F-Strings with Dictionaries

Access dictionary values directly in f-strings:

person = {"name": "Alice", "age": 25}
print(f"{person['name']} is {person['age']} years old.")

Explanation: Dictionary keys can be accessed inside f-strings just like normal variables.

Output:

Alice is 25 years old.

Key Takeaways

• F-strings are prefixed with f and allow direct embedding of variables and expressions.

• They provide improved readability and performance compared to .format() and % formatting.

• You can format numbers, dates, and multiline strings efficiently with f-strings in all of your Python projects.

Practice Exercise

Here's a simple challenge, try writing a Python script that asks for a user's name and age, then prints a greeting using f-strings:

name = input("Enter your name: ")
age = int(input("Enter your age: "))
print(f"Hello, {name}! You are {age} years old.")

Wrapping Up

Python f-strings simplify string formatting while improving readability and efficiency. Whether you're handling numbers, dates, or dictionaries, f-strings are the modern go-to solution for Python developers. Happy coding!