What Are Assertions In Python?

In Python, assertions are statements that allow developers to test the correctness of their code. If the assertion evaluates to true, nothing happens; otherwise, an AssertionError is raised.

Let's look at how assertions are used in Python first, but first let's create a Book class. Below is an example Python code.

from typing import NamedTuple

class Book(NamedTuple):
    author: str
    title: str
    published_year : int

We created a Book class that inherits from the typing module's NamedTuple class. Because NamedTuples are tuples by default, our Book class is immutable.
Creating an instance of the Book class

>>> book1 = Book('Christian Mayer', 'The Art Of Clean Code', 2022)
>>> book1
Book(author='Christian Mayer', title='The Art Of Clean Code', published_year=2022)

Let's use the assert statement to test for the truthiness of the published_year.

>>> assert(book1.published_year == "2022")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError
>>>

AssertionError was raised because the published_year has an integer type and we tested it against a string.

Now let's test it with a valid year value of 2022

>>> assert(book1.published_year == 2022)
>>>

Because, the assertion statement is true nothing happened

Why Use Assertions?

• As a debugging aid rather than an error handling mechanism
• As internal self-checks for a program
• Allows developers to find the likely root cause of a bug more quickly