String formatting is the process of inserting values into a string to create meaningful output.

Before the introduction of str.format() and f-strings, Python commonly used the % operator for string formatting. This style is known as printf-style formatting because it is similar to the printf() function in the C programming language.

The % operator replaces format specifiers such as %s, %d, and %f with actual values.

Although modern Python code often uses f-strings, you may still encounter % formatting in older codebases, making it important to understand.

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Syntax #

"format string" % value

For multiple values:

"format string" % (value1, value2, value3)

Common Format Specifiers #

Specifier	Description
%s	String
%d	Integer
%f	Floating-point number
%.2f	Floating-point number with 2 decimal places
%c	Single character
%%	Prints the % character

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Example 1: Formatting a String #

main.py

name = "Alice"

print("Hello, %s!" % name)

Output #

Hello, Alice!

Explanation #

• %s is the placeholder for a string.
• The value of name replaces %s.
• The formatted string is printed.

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Example 2: Formatting an Integer #

main.py

age = 21

print("Age: %d" % age)

Output #

Age: 21

Explanation #

• %d is used for integers.
• The integer value replaces the placeholder.
• The output displays the formatted integer.

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Example 3: Formatting a Floating-Point Number #

main.py

price = 49.95

print("Price: %f" % price)

Output #

Price: 49.950000

Explanation #

• %f formats a floating-point number.
• By default, six digits are displayed after the decimal point.
• Python automatically formats the number.

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Example 4: Limiting Decimal Places #

main.py

pi = 3.14159265

print("Pi = %.2f" % pi)

Output #

Pi = 3.14

Explanation #

• %.2f displays two digits after the decimal point.
• Python rounds the value automatically.
• This format is useful for displaying measurements and financial values.

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Example 5: Formatting Multiple Values #

main.py

name = "John"
age = 25

print("Name: %s, Age: %d" % (name, age))

Output #

Name: John, Age: 25

Explanation #

• Multiple values are passed as a tuple.
• %s receives the string.
• %d receives the integer.
• Each placeholder is replaced in order.

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Example 6: User Input #

main.py

name = input("Enter your name: ")

print("Welcome, %s!" % name)

Sample Input #

Alice

Output #

Welcome, Alice!

Explanation #

• The user enters a string.
• %s inserts the entered name into the message.
• The formatted greeting is displayed.

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Example 7: Printing a Percent Sign #

main.py

score = 95

print("Score: %d%%" % score)

Output #

Score: 95%

Explanation #

• %% prints a literal percent sign.
• %d formats the integer.
• Without %%, Python would interpret % as another format specifier.

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Example 8: Formatting a Character #

main.py

letter = 'A'

print("Letter: %c" % letter)

Output #

Letter: A

Explanation #

• %c formats a single character.
• The character replaces the placeholder.
• The formatted string is printed.

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Example 9: Mixing Different Data Types #

main.py

name = "Laptop"
quantity = 3
price = 599.99

print("Product: %s, Quantity: %d, Price: $%.2f" % (name, quantity, price))

Output #

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

Explanation #

• %s formats the product name.
• %d formats the quantity.
• %.2f formats the price with two decimal places.
• All placeholders are replaced in sequence.

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Example 10: Checking the Result Type #

main.py

name = "Python"

result = "Welcome %s" % name

print(result)
print(type(result))

Output #

Welcome Python
<class 'str'>

Explanation #

• String formatting creates a new string.
• The formatted result is stored in result.
• Its data type is str.

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Common Mistakes #

1. Using the Wrong Format Specifier #

Incorrect

age = "21"

print("Age: %d" % age)

Output #

TypeError: %d format: a real number is required, not str

Reason

%d expects an integer, but a string was provided.

Correct

age = 21

print("Age: %d" % age)

or

age = "21"

print("Age: %s" % age)

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2. Forgetting Parentheses for Multiple Values #

Incorrect

name = "John"
age = 25

print("Name: %s Age: %d" % name, age)

Output #

TypeError

Reason

Multiple values must be grouped into a tuple.

Correct

print("Name: %s Age: %d" % (name, age))

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3. Forgetting to Escape the Percent Sign #

Incorrect

score = 80

print("Score: %d%" % score)

Output #

ValueError: incomplete format

Reason

A single % starts another format specifier.

Correct

print("Score: %d%%" % score)

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4. Expecting %f to Show Only Two Decimal Places #

Incorrect

price = 19.5

print("Price: %f" % price)

Incorrect Expectation #

Price: 19.50

Actual Output #

Price: 19.500000

Reason

%f displays six decimal places by default.

Correct

print("Price: %.2f" % price)

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Best Practices #

• Use the correct format specifier for each data type.
• Use %.2f when displaying floating-point values with two decimal places.
• Group multiple values into a tuple.
• Use %% to print a percent sign.
• Prefer f-strings for new Python programs, but understand % formatting for maintaining older code.

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Key Points to Remember #

• % formatting is known as printf-style formatting.
• %s formats strings.
• %d formats integers.
• %f formats floating-point numbers.
• %.2f limits floating-point output to two decimal places.
• Multiple values are passed as a tuple.
• % formatting creates and returns a new string.