Python str.format() – Format Strings Easily with Placeholders

Python str.format() Method

The str.format() method in Python lets you insert values into a string using curly braces {} as placeholders. It’s a powerful and flexible way to build dynamic strings.

Syntax

"string with {} placeholders".format(values...)

Parameters:

  • values... – One or more values to be inserted into the string in the order of the placeholders.

Returns:

  • A new string with the placeholders replaced by the provided values.

Example 1: Basic Usage

name = "Alice"
message = "Hello, {}!".format(name)
print(message)
Hello, Alice!

Example 2: Multiple Placeholders

name = "Bob"
age = 30
intro = "My name is {} and I am {} years old.".format(name, age)
print(intro)
My name is Bob and I am 30 years old.

Example 3: Using Index Numbers

sentence = "The {1} is {0} years old.".format(10, "car")
print(sentence)
The car is 10 years old.

Why? The numbers inside {} refer to the index of values passed into format().

Example 4: Named Placeholders

info = "Name: {name}, Age: {age}".format(name="Charlie", age=25)
print(info)
Name: Charlie, Age: 25

Example 5: Formatting Numbers

price = 49.99
text = "Price: ${:.2f}".format(price)
print(text)
Price: $49.99

Why? :.2f formats the float to 2 decimal places.

Common Use Cases

  • Displaying user data in templates or messages
  • Logging or outputting debug information
  • Formatting numbers, dates, or currency

Common Mistakes

  • Mismatch between number of placeholders and arguments
  • Using incompatible types (e.g., formatting a list without converting)

Interview Tip

In coding interviews, use str.format() when building readable strings, especially in questions involving string construction or reporting.

Summary

  • str.format() lets you insert variables into strings using {}
  • You can use automatic order, index numbers, or named placeholders
  • You can apply formatting inside the placeholders

Practice Problem

Write a program that takes a name and score from the user and prints a sentence using str.format():

name = input("Enter your name: ")
score = input("Enter your score: ")
print("Hello, {}. You scored {} points.".format(name, score))

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