f-strings in Python

January 8, 2021 3 minutes read

F-strings is a string formatting mechanism introduced by PEP 498 for Python versions 3.6 and above.

Python supports multiple ways to format text strings.

% formatting
.format()
string.Template

Each of these methods have their advantages, but in addition have disadvantages that make them cumbersome to use in practice. This PEP proposed to add a new string formatting mechanism: Literal String Interpolation, also called f-strings(because of the leading f character preceding the string literal).

Create an f-string

To create an f-string, prefix a string with the letter f. It works similarly to str.format().

Code

num1 = 10
num2 = 5
  
# Adding both numbers 
sum = num1 + num2 
  
# printing values 
print(f"Sum of {num1} and {num2} is {sum}")

Output

Sum of 10 and 5 is 15

Embed Python expressions in f-string

f-strings provide a convenient way to embed python expressions inside string literals for formatting.

Code

num1 = 10
num2 = 5
  
# printing values 
print(f"Sum of {num1} and {num2} is {num1+num2}")

Output

Sum of 10 and 5 is 15

You can also call Python functions within a f-string.

Code

x = "Pylenin"

print(f"{x.lower()}")

Output

pylenin

You could also work with objects created from classes with f-strings.

Code

class Programmer:
    def __init__(self, name, language):
        self.name = name
        self.language = language

    def __str__(self):
        return f"{self.name} like {self.language}"

new_programmer = Programmer('Pylenin', 'Python')

print(f"{new_programmer}")

Output

Pylenin like Python

Multiline f-strings

You can use multiple lines with f-strings. Just remember to put f in fron of every line.

Code

name = "Pylenin"
language="Python"
age = 29

str1 = f"Hello, I am {name}. \n"\
       f"I love {language}. \n"\
       f"I am {age} years old."

print(str1)

Output

Hello, I am Pylenin. 
I love Python. 
I am 29 years old.

f-strings are fast

Since f-strings are evaluated at runtime, they are faster than both %-formatting, String Template formatting and string.format() method.

You can confirm it by using the timeit module in Python.

Example 1 - Timer with % operator

from timeit import timeit

str1 = """
name = "Pylenin"
language="Python"
age = 29

str1 = "Hello, I am %s. "\
       "I love %s. "\
       "I am %d years old."%(name, language, age)

print(str1)
"""
print(timeit(str1, number=10000))

The above code took 0.2817505 secs.

Example 2 - timeit for string.format() method

from timeit import timeit

str1 = """
name = "Pylenin"
language="Python"
age = 29

str1 = "Hello, I am {0}. "\
       "I love {1}. "\
       "I am {2} years old.".format(name, language, age)

print(str1)
"""
print(timeit(str1, number=10000))

The above code took 0.3652804 secs.

Example 3 - timeit for String Template Class

from timeit import timeit

str1 = """
from string import Template

name = "Pylenin"
language="Python"
age = 29

str1 = Template("Hello, I am $name. "\
                "I love $language. "\
                "I am $age years old.")

print(str1.substitute(name=name, language=language, age=age))
"""
print(timeit(str1, number=10000))

The above code took 0.1888825 secs.

Example 4 - timeit for f-strings

from timeit import timeit

str1 = """
name = "Pylenin"
language="Python"
age = 29

str1 = f"Hello, I am {name}. "\
       f"I love {language}. "\
       f"I am {age} years old."

print(str1)
"""
print(timeit(str1, number=10000))

With f-strings, the code only took 0.1565294 secs, considerably lesser than any other method.

Comparison of Execution time for all String Formatting Methods in Python

Python offers 3 other ways of String Formatting. Check out the definite guide to Python String Formatting

Create an f-string

Embed Python expressions in f-string

Multiline f-strings

f-strings are fast

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