How to Use Python f-strings: Writing Variable Embedding and Expression Evaluation Concisely

Introduced in Python 3.6, f-strings (formatted string literals) are currently the most recommended way to embed variables and calculations into strings.

Compared to the traditional .format() method or the % operator, f-strings reduce the amount of code required and significantly improve readability. They also have the advantage of faster execution speed.

This article explains everything from the basic usage of f-strings to advanced features like expression evaluation and formatting.

Basic Syntax of f-strings

To use an f-string, add f or F immediately before the string’s starting quote (" or '). Then, write variable names or expressions inside {} (curly braces) within the string.

Syntax:

f"String {variable_name} String"

Specific Usage Example: Inventory Management System

Here is an example program that displays a product name and its stock count.

product_name = "Gaming Monitor"
current_stock = 15

# Embed variables using an f-string
status_message = f"Product Name: {product_name} | Stock: {current_stock}"

print(status_message)

Execution Result:

Product Name: Gaming Monitor | Stock: 15

You can see that the variables are automatically converted to strings and embedded in the specified positions.

Expression Evaluation and Method Calls inside {}

A powerful feature of f-strings is that you can write Python expressions (calculations) and method calls directly inside the {} brackets, rather than just simple variables.

Embedding Calculations

You can calculate the total inventory value from the unit price and stock count and display it on the spot.

unit_price = 25000
stock = 4

# Perform multiplication inside {}
total_value_msg = f"Total Value: {unit_price * stock} yen"

print(total_value_msg)

Execution Result:

Total Value: 100000 yen

Method Calls

It is also possible to display the result of a method execution, such as a string method.

code = "item-a001"

# Call the .upper() method to convert to uppercase
formatted_code = f"Product Code: {code.upper()}"

print(formatted_code)

Execution Result:

Product Code: ITEM-A001

Formatting

In f-strings, you can control number separators and decimal places by writing a format specifier after a colon in the format {value:format}.

price = 12800
tax_rate = 0.1

# 1. Comma separator (:,)
# 2. Display up to 1 decimal place (:.1f)
price_info = f"Price: {price:,} yen (Inc. Tax: {price * (1 + tax_rate):,.1f} yen)"

print(price_info)

Execution Result:

Price: 12,800 yen (Inc. Tax: 14,080.0 yen)

Summary

• Add f before the quote (e.g., f"...") to create an f-string.
• Embed variable values using {variable_name}.
• Embed calculation results or method return values using {expression}.
• Specify display formats (digit separators, decimal places) using {value:format}.

If you are using Python 3.6 or later, it is recommended to prioritize f-strings over .format() for better readability and performance.