String Interpolation in Python

1. String Interpolation Using the Modulo (%) in Python
2. String Interpolation Using the format() Method in Python
3. String Interpolation Using the Formatted Strings (f-strings) in Python
4. String Interpolation Using the Template Class in Python
5. Summary

String interpolation refers to the technique of inserting values of variables in place of placeholders in a string. Using string interpolation, one can dynamically insert values inside a string.

In Python, there are four ways in which we can perform string interpolation, namely, modulo (%), format() method, formatted strings, and template class. In this article, we will learn in detail about these methods.

String Interpolation Using the Modulo (%) in Python

We can use the modulo (%) for formatting strings in Python. This approach is similar to the printf() in the C programming language.

Following are the placeholders we can use inside strings for formatting.

%d = Integer
%f = Float
%s = String
%x = Hexadecimal
%o = Octal

Note that it is necessary to place a modulo character before characters inside strings to be considered placeholders. Following is the syntax for using modulo (%).

"<string with placeholders>" % ( < comma separated values or variables > )

Now, we’re done with some brief introduction; let us understand how to use this concept practically with the help of some examples. Refer to the following Python code for this.

a = 2000
b = "Hi"
c = "Python"
d = True
e = 3.14
print("%d + 1000 = 3000" % (a))
print("%s is a web framework written in %s" % ("Django", c))
print("[%d, %s, %s, %s, %f]" % (a, b, c, d, e))
print("%s! My favourite programming language is %s" % (b, c))
print("The value of PI or π: %f" % (e))

Output:

2000 + 1000 = 3000
Django is a web framework written in Python
[2000, Hi, Python, True, 3.140000]
Hi! My favourite programming language is Python
The value of PI or π: 3.140000

Note how placeholders are used inside strings for formatting and what placeholder is used for what kind of value.

String Interpolation Using the format() Method in Python

The format() method can be used for formatting strings in Python. This method is similar to the previous one but here, {} acts as a placeholder for every type of value.

Following is the syntax for the format() method.

"<string with placeholdes>".format( < comma separated values and variables > )

Refer to the following Python code to understand this concept better with the help of some relevant examples.

a = 2000
b = "Hi"
c = "Python"
d = True
e = 3.14
print("{} + 1000 = 3000".format(a))
print("{} is a web framework written in {}".format("Django", c))
print("[{}, {}, {}, {}, {}]".format(a, b, c, d, e))
print("{}! My favourite programming language is {}".format(b, c))
print("The value of PI or π: {}".format(e))

Output:

2000 + 1000 = 3000
Django is a web framework written in Python
[2000, Hi, Python, True, 3.14]
Hi! My favourite programming language is Python
The value of PI or π: 3.14

String Interpolation Using the Formatted Strings (f-strings) in Python

Formatted strings are unique strings in Python. When prefixed with an f character, regular strings are formatted strings.

Formatted strings are also known as f-strings. These strings are used to dynamically insert string representations of variables and objects inside strings.

We can add {} inside strings, and inside these blocks, we can add variables or logic that return some value.

Following is the syntax for formatted strings.

f"{<variable>} {<variable>} {<variable>}"

Let’s understand this concept using some relevant examples. Refer to the following Python code for the same.

def hello():
    return "Hello"


a = 2000
b = "Hi"
c = "Python"
d = True
e = 3.14
print(f"{a} + 1000 = 3000")
print(f"Django is a web framework written in {c}")
print(f"[{a}, {b}, {c}, {d}, {e}]")
print(f"{hello()}! My favourite programming language is {c}")
print(f"The value of PI or π: {e}")
print(f"1000 + 2000 = {1000 + 2000}")
print(f"10 * 20 - 25 = {10 * 20 - 25}")

Output:

2000 + 1000 = 3000
Django is a web framework written in Python
[2000, Hi, Python, True, 3.14]
Hello! My favourite programming language is Python
The value of PI or π: 3.14
1000 + 2000 = 3000
10 * 20 - 25 = 175

String Interpolation Using the Template Class in Python

Python has an in-built module, string, a class Template that allows us to create dynamic strings. Using this method, we can perform $- based substitutions.

For example, for the Hello $name string, name is a variable, and we can provide a value for this variable. We can use the substitute() of the Template class to substitute values.

This method accepts a dictionary, where variables of the template string are keys, and they point to values. Instead of using a dictionary, we can also provide keyword arguments.

Note that the Python interpreter will throw a KeyError if the key is not found. To avoid that, one can either make sure that the required keys exist in the dictionary or use the safe_substitute() method to leave the string unchanged for that placeholder.

Now that we are done with some theory, let us understand this concept with the help of some relevant examples. Refer to the following Python code for the same.

from string import Template


def pi():
    return 3.14


t1 = Template("$a + 1000 = 3000")
t2 = Template("$package is a web framework written in $language")
t3 = Template("$greeting! My favourite programming language is $language")
t4 = Template("[$a, $b, $c, $d, $e]")
t5 = Template("The value of PI or π: $pi")
print(t1.substitute({"a": 2000}))
print(t2.substitute({"package": "Flask", "language": "Python"}))
print(t3.substitute({"greeting": "Hey", "language": "Python"}))
print(t4.substitute(a=2000, b="Hi", c="Python", d=True, e=999.909))
print(t5.substitute(pi=pi()))

Output:

2000 + 1000 = 3000
Flask is a web framework written in Python
Hey! My favourite programming language is Python
[2000, Hi, Python, True, 999.909]
The value of PI or π: 3.14

Refer to the following Python code to use the safe_substitute() method.

from string import Template


def pi():
    return 3.14


t1 = Template("$a + 1000 = 3000")
t2 = Template("$package is a web framework written in $language")
t3 = Template("$greeting! My favourite programming language is $language")
t4 = Template("[$a, $b, $c, $d, $e]")
t5 = Template("The value of PI or π: $pi")
print(t1.safe_substitute({}))
print(t2.safe_substitute({"package": "Flask"}))
print(t3.safe_substitute({"language": "Python"}))
print(t4.safe_substitute(c="Python", d=True, e=999.909))
print(t5.safe_substitute())

Output:

$a + 1000 = 3000
Flask is a web framework written in $language
$greeting! My favourite programming language is Python
[$a, $b, Python, True, 999.909]
The value of PI or π: $pi

Summary

Here are some important points about the above methods we used.

1. The f-strings method is the most powerful and easiest method that also increases the readability of the code.
2. %-format method is an old method and not recommended to use because it decreases the readability of the code.
3. In str.format() we need to pass the string object the format() function for string interpolation.
4. In Template strings, we create templates by importing the Template class from the built-in string module.

In this tutorial, we discussed four different methods that can be used for string interpolation. We also learned the syntax and how to use all of these methods in our application without any problems.