Python String Interpolation

String Interpolation is the process of substituting values of variables into placeholders in a string. Let’s consider an example to understand it better, suppose you want to change the value of the string every time you print the string like you want to print “hello <name> welcome to geeksforgeeks” where the <name> is the placeholder for the name of the user. Instead of creating a new string every time, string interpolation in Python can help you to dynamically change the placeholder with the name of the user. 

% – Formatting

% – Formatting is a feature provided by Python which can be accessed with a % operator. This is similar to printf style function in C.

Example: Formatting string using % operator

Python3

# Python program to demonstrate
# string interpolation


n1 = 'Hello'
n2 = 'GeeksforGeeks'

# for single substitution
print("Welcome to % s" % n2)

# for single and multiple substitutions()
# mandatory
print("% s ! This is % s." % (n1, n2))

Output

Welcome to GeeksforGeeks
Hello ! This is GeeksforGeeks.

Let’s say it’s just a complicated version, but we can use it if we have a lot of variables to get substituted in the string as we don’t always want to use(“string” + variable + “string” + variable + variable + “string”) this representation. So for this purpose, we can go with %-formatting.

Note: To know more about %-formatting, refer to String Formatting in Python using %

Str.format()

str.format()work by putting in one or more replacement fields and placeholders defined by a pair of curly braces { } into a string. The value we wish to put into the placeholders and concatenate with the string passed as parameters into the format function. 

Example: Formatting strings using format() method

Python3

# Python program to demonstrate
# string interpolation


n1 = 'Hello'
n2 = 'GeeksforGeeks'

# for single substitution
print('{}, {}'.format(n1, n2))

Output

Hello, GeeksforGeeks

We can also use the variable name inside the curly braces {}. This will allow us to use the parameters of format functions in any order we want.

Example: Format functions with variables inside curly braces.

Python3

n1 = "Hello"
n2 = "GeeksForGeeks"

# for single or multiple substitutions
# let's say b1 and b2 are formal parameters
# and n1 and n2 are actual parameters
print("{b1}! This is {b2}.".format(b1=n1, b2=n2))

# we can also change the order of the
# variables in the string without changing
# the parameters of format function
print("{b2}! This is {b1}.".format(b1=n1, b2=n2))

Output

Hello! This is GeeksForGeeks.
GeeksForGeeks! This is Hello.

Note: To know more about str.format(), refer to format() function in Python

f-strings

PEP 498 introduced a new string formatting mechanism known as Literal String Interpolation or more commonly as F-strings (because of the leading f character preceding the string literal). The idea behind f-strings is to make string interpolation simpler. 

To create an f-string, prefix the string with the letter “ f ”. The string itself can be formatted in much the same way that you would with str.format(). F-strings provide a concise and convenient way to embed python expressions inside string literals for formatting.

Example: Formatting Strings using f-strings

Python3

# Python program to demonstrate
# string interpolation


n1 = 'Hello'
n2 = 'GeeksforGeeks'

# f tells Python to restore the value of two
# string variable name and program inside braces {}
print(f"{n1}! This is {n2}")

Output

Hello! This is GeeksforGeeks
(2 * 3)-10 = -4

We can also use f-strings to calculate some arithmetic operations and it will perform the inline arithmetic. See the below example – 

Example: Inline arithmetic using f-strings

Python3

a = 2
b = 3
c = 10

print(f"({a} * {b})-{c} = {(2 * 3)-10}")

Output

(2 * 3)-10 = -4

Note: To know more about f-strings, refer to f-strings in Python

String Template Class

In the String module, Template Class allows us to create simplified syntax for output specification. The format uses placeholder names formed by $ with valid Python identifiers (alphanumeric characters and underscores). Surrounding the placeholder with braces allows it to be followed by more alphanumeric letters with no intervening spaces. Writing $$ creates a single escaped $:

Example: Formatting string using Template Class

Python3

# Python program to demonstrate
# string interpolation


from string import Template

n1 = 'Hello'
n2 = 'GeeksforGeeks'

# made a template which we used to
# pass two variable so n3 and n4
# formal and n1 and n2 actual
n = Template('$n3 ! This is $n4.')

# and pass the parameters into the template string.
print(n.substitute(n3=n1, n4=n2))

Output

Hello ! This is GeeksforGeeks.

Note: To know more about String Template class, refer to String Template Class in Python