How to perform string interpolation in Python

String interpolation is a technique used in various programming languages to embed expressions or variables within string literals. It allows you to construct strings by combining static text with dynamic content. The resulting string is formed by replacing placeholders or variables with their actual values.

Python offers several ways to perform string interpolation:

• Use f-strings, generally considered the most concise and efficient way.
• Use format() method when you need more flexibility or using Python older than 3.6.
• Use string Template class with custom patterns to handle complex formatting logic.
• Use %-formatting only for historical compatibility.

Each method has its own advantages and disadvantages.

1. Using f-strings (formatted string literals), a feature introduced in Python 3.6 that allows for concise and readable string formatting. This method uses f or F as prefix and curly braces {} to indicate placeholders and embed expressions directly inside the string.

name = "John"
age = 30

message = f"My name is {name} and I am {age} years old."
print(message)  
# My name is John and I am 30 years old.

area = 5 * 7
print(f"The area of the rectangle is {area} square meters.")
# The area of the rectangle is 35 square meters.

There are many different format specifiers, written after a colon (:) inside the curly braces that enclose the expression, that can be used with f-strings. They can be used to specify the type, precision, alignment, padding, grouping, and other aspects of the formatting.

name = "Alice"
age = 25
print(f"Hello, {name}. You are {age:03d} years old.")
# Hello, Alice. You are 025 years old.

2. Using format() method, a versatile option available in all Python versions. It allows you to define placeholders in the string using curly braces and pass the values as arguments to the format() method.

name = "John"
age = 30

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

area = 5 * 7
print("The area of the rectangle is {0} square meters.".format(area))
# The area of the rectangle is 35 square meters.

3. Using %-formatting (older style), the older method and less readable than f-strings and the format() method. It uses placeholders with a % symbol and format specifiers like s (string), d (integer), and f (float) to define the type of value to be inserted.

name = "John"
age = 30

message = "My name is %s and I am %d years old." % (name, age)
print(message)  
# My name is John and I am 30 years old.

area = 5 * 7
print("The area of the rectangle is %.2f square meters." % (area))
# The area of the rectangle is 35.00 square meters.

4. Using string template. This method uses Template class from the string module and dollar signs $ to indicate placeholders and the substitute() method to pass the values.

from string import Template
name = "David"
age = 40
t = Template("Hello, $name. You are $age years old.")
print(t.substitute(name=name, age=age))
# Output: Hello, David. You are 40 years old.