An f-string (formatted string literal) is the modern and recommended way to format strings in Python.

An f-string is created by placing the letter f or F immediately before the opening quotation mark. Expressions enclosed within curly braces ({}) are evaluated and their values are inserted directly into the string.

Compared to % formatting and the format() method, f-strings are easier to read, require less code, and generally execute faster.

F-strings are widely used in modern Python programs for displaying variables, calculations, and expressions.

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Syntax #

f"Text {expression}"

or

F"Text {expression}"

Components #

Component	Description
f or F	Prefix indicating a formatted string literal.
{}	Placeholder containing a variable or expression.
expression	Variable, value, or expression evaluated by Python.

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Example 1: Formatting a Variable #

main.py

name = "Alice"

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

Output #

Hello, Alice!

Explanation #

• The f prefix creates an f-string.
• {name} is replaced by the value of the variable name.
• The formatted string is printed.

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Example 2: Formatting Multiple Variables #

main.py

name = "John"
age = 25

print(f"Name: {name}, Age: {age}")

Output #

Name: John, Age: 25

Explanation #

• Both variables are placed inside curly braces.
• Python replaces each placeholder with its corresponding value.
• The resulting string is displayed.

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Example 3: Using Expressions #

main.py

a = 10
b = 20

print(f"Sum = {a + b}")

Output #

Sum = 30

Explanation #

• The expression a + b is evaluated.
• Python inserts the result into the string.
• Expressions can be written directly inside {}.

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Example 4: Performing Calculations #

main.py

length = 5
width = 8

print(f"Area = {length * width}")

Output #

Area = 40

Explanation #

• The multiplication expression is evaluated.
• The computed result replaces the placeholder.
• No separate variable is required.

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Example 5: User Input #

main.py

name = input("Enter your name: ")

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

Sample Input #

Alice

Output #

Welcome, Alice!

Explanation #

• The user enters a name.
• The value stored in name is inserted into the f-string.
• The greeting is displayed.

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Example 6: Mixing Different Data Types #

main.py

product = "Laptop"
quantity = 3
price = 599.99

print(f"Product: {product}, Quantity: {quantity}, Price: ${price}")

Output #

Product: Laptop, Quantity: 3, Price: $599.99

Explanation #

• Strings, integers, and floating-point numbers can all be inserted.
• Python automatically converts values to string form when necessary.
• The output is easy to read.

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Example 7: Calling Functions Inside an f-String #

main.py

word = "python"

print(f"{word.upper()}")

Output #

PYTHON

Explanation #

• Function calls can be placed inside {}.
• upper() converts the string to uppercase.
• The returned value is inserted into the string.

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Example 8: Using an Uppercase Prefix #

main.py

language = "Python"

print(F"Learning {language}")

Output #

Learning Python

Explanation #

• Both f and F create formatted string literals.
• They behave exactly the same.
• The choice is based on coding style.

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Example 9: Formatting a Boolean Expression #

main.py

age = 18

print(f"Eligible: {age >= 18}")

Output #

Eligible: True

Explanation #

• The comparison age >= 18 is evaluated.
• The Boolean result is inserted into the string.
• Expressions inside {} can return any data type.

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Example 10: Checking the Result Type #

main.py

name = "Python"

result = f"Welcome {name}"

print(result)
print(type(result))

Output #

Welcome Python
<class 'str'>

Explanation #

• An f-string creates a new string.
• The formatted result is stored in result.
• Its data type is str.

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Common Mistakes #

1. Forgetting the f Prefix #

Incorrect

name = "Alice"

print("Hello, {name}")

Output #

Hello, {name}

Reason

Without the f prefix, Python treats {name} as ordinary text.

Correct

print(f"Hello, {name}")

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2. Using Undefined Variables #

Incorrect

print(f"Hello, {username}")

Output #

NameError: name 'username' is not defined

Reason

The variable inside {} must already exist.

Correct

username = "Alice"

print(f"Hello, {username}")

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3. Forgetting the Curly Braces #

Incorrect

name = "Alice"

print(f"Hello, name")

Output #

Hello, name

Reason

Variables must be enclosed within {}.

Correct

print(f"Hello, {name}")

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4. Assuming an f-String Changes the Variable #

Incorrect

name = "Alice"

f"Hello, {name}"

print(name)

Output #

Alice

Reason

An f-string creates a new string.
It does not modify the original variable.

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Best Practices #

• Prefer f-strings over % formatting and format() in modern Python programs.
• Use meaningful variable names inside f-strings.
• Keep expressions simple for better readability.
• Use f-strings for messages, reports, and user-friendly output.
• Remember to include the f prefix before the opening quotation mark.

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Key Points to Remember #

• An f-string is created using the f or F prefix.
• Values inside {} are evaluated automatically.
• Variables, expressions, and function calls can be placed inside {}.
• F-strings produce a new string of type str.
• F-strings are more readable than % formatting and format().
• The f prefix is mandatory.
• F-strings are the recommended formatting method in modern Python.