Python f-Strings

What Are f-Strings?

An f-string is a Python string that allows you to insert variables and expressions directly inside the string.

The letter f is placed before the opening quotation mark.

name = "Adnan"

message = f"Hello, {name}!"

print(message)

Hello, Adnan!

Why Use f-Strings?

Before f-strings, programmers commonly used concatenation or older formatting techniques.

Concatenation

name = "Adnan"
age = 30

message = "My name is " + name + " and I am " + str(age) + " years old."

print(message)

Using an f-string

name = "Adnan"
age = 30

message = f"My name is {name} and I am {age} years old."

print(message)

Basic f-String Syntax

The general structure is:

f"text {expression} text"

For example:

name = "Sara"

print(f"Welcome, {name}!")

Welcome, Sara!

Using Variables

You can insert variables of different data types directly into an f-string.

name = "Ali"
age = 25
height = 5.9
student = True

print(f"Name: {name}")
print(f"Age: {age}")
print(f"Height: {height}")
print(f"Student: {student}")

Name: Ali
Age: 25
Height: 5.9
Student: True

Python automatically converts the values into their string representation for the formatted output.

Using Multiple Values

An f-string can contain as many replacement fields as your output requires.

product = "Laptop"
price = 85000
quantity = 2

print(f"Product: {product}")
print(f"Price: Rs. {price}")
print(f"Quantity: {quantity}")
print(f"Total: Rs. {price * quantity}")

Product: Laptop
Price: Rs. 85000
Quantity: 2
Total: Rs. 170000

Using Expressions Inside f-Strings

One of the most powerful features of f-strings is that you can place Python expressions inside the braces.

a = 10
b = 5

print(f"Sum: {a + b}")
print(f"Difference: {a - b}")
print(f"Product: {a * b}")
print(f"Division: {a / b}")

Sum: 15
Difference: 5
Product: 50
Division: 2.0

You can also use comparisons and conditional expressions.

age = 20

print(f"Status: {'Adult' if age >= 18 else 'Minor'}")

Status: Adult

Calling Functions Inside f-Strings

You can call functions inside replacement fields.

name = "python"

print(f"Language: {name.title()}")
print(f"Length: {len(name)}")

Language: Python
Length: 6

This becomes especially useful when formatting data before displaying it to users.

Format Specifiers

f-strings allow you to control how values are displayed using a format specifier.

f"{value:format_specifier}"

The colon separates the expression from its formatting instructions.

price = 1250.5678

print(f"Price: {price:.2f}")

Price: 1250.57

Formatting Decimal Numbers

Use .2f when you want a number displayed with exactly two digits after the decimal point.

price = 1999.5
tax = 347.2567

print(f"Price: {price:.2f}")
print(f"Tax: {tax:.2f}")

Price: 1999.50
Tax: 347.26

The number after the decimal specifies how many decimal places should be displayed.

number = 12.3456789

print(f"{number:.1f}")
print(f"{number:.2f}")
print(f"{number:.3f}")

12.3
12.35
12.346

Thousands Separators

Large numbers can be made easier to read using the comma formatting option.

population = 240000000

print(f"Population: {population:,}")

Population: 240,000,000

This is especially useful for reports, financial dashboards, sales systems, and business applications.

Formatting Percentages

The % format specifier converts a decimal value into a percentage representation.

completion = 0.875

print(f"Course completion: {completion:.1%}")

Course completion: 87.5%

The original value is 0.875, but Python displays it as 87.5%.

Text Alignment

f-strings can align text to the left, right, or center.

name = "Python"

print(f"{name:<15}")
print(f"{name:>15}")
print(f"{name:^15}")

Python         
         Python
    Python     

The symbols mean:

• < — left aligned
• > — right aligned
• ^ — centered

Width & Padding

You can specify a minimum field width when formatting values.

item = "Laptop"
price = 85000

print(f"{item:<15} {price:>10,}")

Laptop             85,000

This technique can be useful when creating simple text-based tables and reports.

Debugging with the = Feature

Modern Python versions provide a convenient f-string debugging feature using =.

name = "Ali"
age = 25

print(f"{name=}")
print(f"{age=}")
print(f"{age + 5=}")

name='Ali'
age=25
age + 5=30

Quotes & Curly Braces

Sometimes your text needs quotation marks or literal curly braces.

Using Quotes

name = "Ali"

print(f'{name} said "Hello!"')

Ali said "Hello!"

Displaying Literal Curly Braces

Curly braces normally represent replacement fields. To display actual braces, use double braces.

name = "Python"

print(f"{{{name}}}")

{Python}

Practical Real-World Examples

Example 1 — Student Result

name = "Sara"
marks = 87
total = 100

percentage = marks / total

print(f"Student: {name}")
print(f"Marks: {marks}/{total}")
print(f"Percentage: {percentage:.1%}")

Student: Sara
Marks: 87/100
Percentage: 87.0%

Example 2 — Shopping Bill

item = "Keyboard"
price = 3500
quantity = 2

subtotal = price * quantity
tax = subtotal * 0.05
total = subtotal + tax

print(f"Item: {item}")
print(f"Quantity: {quantity}")
print(f"Subtotal: Rs. {subtotal:,}")
print(f"Tax: Rs. {tax:,.2f}")
print(f"Total: Rs. {total:,.2f}")

Item: Keyboard
Quantity: 2
Subtotal: Rs. 7,000
Tax: Rs. 350.00
Total: Rs. 7,350.00

Example 3 — User Profile

name = "Muhammad Adnan"
age = 30
city = "Multan"
profession = "Python Learner"

profile = (
    f"Name: {name}\n"
    f"Age: {age}\n"
    f"City: {city}\n"
    f"Profession: {profession}"
)

print(profile)

Name: Muhammad Adnan
Age: 30
City: Multan
Profession: Python Learner

Example 4 — Simple Sales Report

sales = 1250000
growth = 0.184

print(f"Monthly Sales: Rs. {sales:,}")
print(f"Growth Rate: {growth:.1%}")

Monthly Sales: Rs. 1,250,000
Growth Rate: 18.4%

Common f-String Mistakes

Practice Exercises

Try these exercises yourself before looking for solutions.

Mini Challenge — Professional Invoice

Build a small invoice generator using f-strings.

Your program should:

• Store a customer name.
• Store product name, price, and quantity.
• Calculate the subtotal.
• Calculate a 5% tax.
• Calculate the final total.
• Format large numbers with commas.
• Display monetary values with two decimal places.
• Produce a clean invoice using f-strings.

customer = "Ali"
product = "Wireless Mouse"
price = 2500
quantity = 3

subtotal = ...
tax_rate = ...
tax = ...
total = ...

print("=" * 40)
print(f"Customer: {customer}")
print(f"Product:  {product}")
print(f"Quantity: {quantity}")
print(f"Price:    Rs. {price:,.2f}")
print(f"Subtotal: Rs. {subtotal:,.2f}")
print(f"Tax:      Rs. {tax:,.2f}")
print(f"Total:    Rs. {total:,.2f}")
print("=" * 40)

Quick Knowledge Check

Lesson Summary

f-strings are one of the most useful and readable ways to create dynamic text in modern Python.

• An f-string begins with f before the opening quotation mark.
• Variables and expressions are placed inside { }.
• f-strings can evaluate arithmetic and other Python expressions.
• Functions can be called inside replacement fields.
• Format specifiers control how values are displayed.
• :.2f displays two decimal places.
• , adds thousands separators.
• :.1% displays a percentage with one decimal place.
• <, >, and ^ control text alignment.
• {variable=} is useful for debugging.
• Double curly braces can display literal curly braces.