Assert statement in Python

Assertions are used to make sure that certain conditions are met and that the program execution is proceeding as expected. They  are meant for verifying program correctness and upholding assumptions made in the program. This makes them useful for  debugging and testing purposes as they allow you to monitor variables and invariants at different stages of execution.

In Python, we use the assert keyword to create assertion statements. The keyword , checks whether a condition evaluates to True or False , it throws an AssertionError if False.

The syntax of the assert statement is as shown below:

assert <condition>, <message>
copycopied!

Assert-usage and examples

def divide(a, b):
    assert b != 0
    print(f"{a} / {b} = {a/ b}")

divide(10, 2)
divide(10, 0)
 Visualize 
copycopied!

Output:
10 / 2 = 5.0

Traceback (most recent call last):
  File "<string>", line 6, in <module>
  File "<string>", line 2, in divide
AssertionError

[Finished in 0.010745828039944172s]

In the above example, we defined a function to divide two numbers. The assert statement is meant to verify that the value of b is not equal to 0 in which case the division operation would be undefined.

In the following example, we specify a message in the assert statement that will be displayed if the assertion fails.

def divide(a, b):
    assert b != 0, "Can't divide by 0"
    print(f"{a} / {b} = {a/ b}")

divide(10, 2)
divide(10, 0)
 Visualize 
copycopied!

Output:
10 / 2 = 5.0

Traceback (most recent call last):
  File "<string>", line 6, in <module>
  File "<string>", line 2, in divide
AssertionError: Can't divide by 0

[Finished in 0.009991337079554796s]

Catching an AssertionError exception

The AssertionError exception can be handled just like any other exception including by use of the try/except statement. We can,  put the code that may raise an AssertionError in a try block, then put the code that should run if an AssertionError is raised in the corresponding except block.

import math

def square_root(x):
    assert x > 0 , "x can't be negative"
    return math.sqrt(x)

try:
    square_root(9)
    square_root(16)
    square_root(-25)
except AssertionError as e:
    print(e)
 Visualize 
copycopied!

Output:
x can't be negative
[Finished in 0.009892639238387346s]

In the above example, we used the try/except statement so that when an AssertionError is raised, we display a friendly message rather than the message crashing as it normally would.