Python - Assertions

The process of identifying and fixing bugs in a program is called Debugging.

A very common way of debugging is to use the print() statement. However, after fixing the bug, we must delete the extra print() statements that were added for debugging.

If we forget to delete them, these statements keep executing at runtime, which creates performance problems and disturbs the console output.

To overcome the problem of print() debugging, Python provides the assert statement.

After fixing a bug, we are not required to delete the assert statements — based on our requirement, we can enable or disable them.

The main purpose of assertions is to perform debugging.

Debugging is usually performed in development or test environments, but not in the production environment.

Hence, the assertions concept is applicable only for development and test environments, but not for the production environment.

The conditional_expression is evaluated by the Python interpreter.

• If the condition is True, the program continues normally.
• If the condition is False, the program is terminated by raising an AssertionError.

By seeing the AssertionError, the programmer can analyze the code and fix the problem.

Type	Syntax
Simple Version	assert conditional_expression
Augmented Version	assert conditional_expression, message

In the simple version, only a conditional_expression is given. If the condition is False, an AssertionError is raised without any message.

AssertionError

In the augmented version, a message is also provided along with the conditional_expression. If the condition is False, an AssertionError is raised and the message is displayed.

AssertionError: x must be greater than 100

Write a function that returns the square of a number, and verify it using assert statements. Initially, the function is written incorrectly using the exponent operator:

AssertionError: The square of 3 should be 9

Here, squareIt(x) uses x**x (x raised to the power x) instead of x*x (x multiplied by x).

• squareIt(2) returns 2**2 = 4 → the first assertion passes.
• squareIt(3) returns 3**3 = 27 → the second assertion fails.

Because the second condition is False, the program terminates with AssertionError: The square of 3 should be 9.

The assert statement helped us detect the bug in the function without adding extra print() statements.

After fixing the function to use x*x, all the assertions pass:

4
9
16

Assertions and Exception Handling are used for different purposes:

Aspect	Assertions	Exception Handling
Purpose	To alert the programmer to resolve development-time errors.	To handle runtime errors.
When It Occurs	Used for debugging internal program logic.	Used to handle errors that occur during program execution.
Example	assert statement	try-except blocks