Python String Formatting, F-Strings, Placeholders, and Modifiers

IT Tech Guide October 09, 2025

Python String Formatting, F-Strings, Placeholders, and Modifiers 

📘 Introduction

String formatting in Python is one of the most essential skills for any programmer. Properly formatted strings make your output readable, professional, and easy to maintain. Python offers multiple methods for string formatting, each with its own advantages. In this guide, we will cover everything from basic placeholders to advanced f-string operations.

By the end of this tutorial, you will understand:

  • How to use placeholders to insert variables in strings
  • F-strings (formatted string literals) and their power
  • Modifiers for alignment, width, precision, and padding
  • The str.format() method and its advanced uses
  • Old-style % formatting and when to use it
  • Performing operations and calling functions inside f-strings
  • Exercises and FAQs for practice

🧩 Placeholders

Placeholders are markers inside strings that tell Python where to insert a variable or value. They are often represented as curly braces {} and are a key component of both f-strings and str.format().

Examples:

fruit = "apple"
quantity = 5
print(f"I bought {quantity} {fruit}s today.")
Output: 
I bought 5 apples today.

Indexed Placeholders with str.format():

print("I like {1} and {0}".format("Java", "Python"))
Output: I like Python and Java

Named Placeholders:

print("Name: {name}, Country: {country}".format(name="John", country="USA"))
Output: Name: John, Country: USA

💡 F-Strings (Formatted String Literals)

F-strings, introduced in Python 3.6, allow embedding expressions directly inside string literals. They are prefixed with f or F and are currently the most readable and efficient way to format strings.

Syntax:

f"Text {expression}"

Example:

name = "Alice"
age = 28
print(f"My name is {name} and I am {age} years old.")
Output: My name is Alice and I am 28 years old.

Operations Inside F-Strings:

F-strings allow you to perform calculations and operations directly inside the curly braces.

a = 5
b = 10
print(f"{a} + {b} = {a + b}")
print(f"Double of {b} is {b*2}")
Output: 
5 + 10 = 15
Double of 10 is 20

Calling Functions in F-Strings:

You can also call functions or methods inside f-strings:

print(f"Uppercase: {'python'.upper()}")
print(f"Length: {len('Python')}")
print(f"Square: {pow(5,2)}")
Output: 
Uppercase: PYTHON
Length: 6
Square: 25

⚙️ Modifiers and Format Specifiers

Modifiers help control the formatting of strings, including alignment, width, padding, precision, and type.

Alignment and Width:

ModifierDescriptionExampleOutput
<Left alignf"{'Hi':<10 code="">Hi
>Right alignf"{'Hi':>10}" Hi
^Center alignf"{'Hi':^10}" Hi
0Pad with zerosf"{5:03}"005

Precision and Type:

TypeDescriptionExampleOutput
dDecimal integer{:d}42
fFixed-point float{:.2f}3.14
bBinary{:b}101010
oOctal{:o}52
x / XHexadecimal{:x}2a
e / EScientific notation{:e}1.000000e+00
%Percentage{:.1%}75.0%
sString{:s}Hello

🧮 str.format() Method

The str.format() method is another way to format strings. It supports positional arguments, named arguments, and advanced formatting options.

print("My name is {} and I am {} years old.".format("Alice", 25))
print("I like {1} and {0}".format("Java", "Python"))
print("Name: {name}, Country: {country}".format(name="John", country="USA"))

📊 Old-Style % Formatting

This is the legacy formatting style using the % operator.

name = "Alice"
age = 25
print("Name: %s, Age: %d" % (name, age))
Output: Name: Alice, Age: 25

🧩 Advanced F-String Examples

# Nested formatting
value = 123.4567
print(f"Value: {value:{10}.{6}}")

# Formatting dates
import datetime
today = datetime.date.today()
print(f"Today's date: {today:%B %d, %Y}")

# Multiline f-string
name = "Alice"
age = 25
print(f"""
Hello, {name}!
You are {age} years old.
""")

💪 Exercises

  1. Create an f-string that prints your full name and age.
  2. Format a float to 3 decimal places using f-strings.
  3. Use str.format() to display a table of 3 students and their grades.
  4. Display the binary, octal, and hexadecimal of the number 42.
  5. Create a multiline f-string greeting message.
  6. Align text left, right, and center in 15-character wide fields.
  7. Call functions like len() and upper() inside an f-string.

❓ FAQs

1. Difference between f-strings and str.format()?

F-strings allow inline expressions and are faster. str.format() is older but still used for compatibility.

2. Can we call functions inside f-strings?

Yes, any function or method can be called inside {}.

3. Is % formatting still used?

It's legacy and not recommended for new projects.

4. How to align text?

Use <, >, ^ with optional width. Example: f"{text:^10}"

5. Can f-strings span multiple lines?

Yes, use triple quotes: f"""Multiline {var}"""

🧭 Conclusion

Python string formatting is essential for readable and professional outputs. F-strings are the most efficient, allowing inline operations, function calls, and advanced formatting. Understanding placeholders, modifiers, and str.format() helps you write clean, maintainable code. This guide equips you to format numbers, strings, and dynamic content effectively. Practice exercises and examples reinforce the concepts and prepare you for real-world Python projects.

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