The assert Statement

The assert statement is a debugging aid that tests a condition inside your program. Think of it as a sanity check: you are asserting that something must be true at that point in the code. If your assertion turns out to be false, Python immediately raises an exception.

Purpose and placement

The textbook explains that assert is used to test an expression in the program code. If the test result is false, an exception is raised. This statement is typically placed at the beginning of a function or right after a function call, to check whether the input is valid before proceeding with the rest of the logic.

Syntax

assert Expression[, arguments]

The keyword assert is followed by an expression. Optionally, you can add a comma and then one or more arguments (usually a string message).

How it works

When Python encounters an assert statement, it evaluates the expression immediately after the keyword. If the expression is true, nothing happens — the program continues normally. If the expression is false, Python raises an AssertionError exception. This exception can be caught and handled using try and except, just like any other exception.

Program 1-1: Use of assert statement

The book gives a concrete example: a function negativecheck() that takes a number and, inside the function, uses an assert statement to check whether the number is non-negative.

print("use of assert statement")
def negativecheck(number):
    assert(number>=0), "OOPS... Negative Number"
    print(number*number)
print (negativecheck(100))
print (negativecheck(-350))

If the number is negative, the assertion fails, an AssertionError is thrown with the message "OOPS... Negative Number", and the subsequent statement inside the function — print(number*number) — is not executed. If the number is non-negative, the assertion silently passes and the square is printed as normal.

The output shown in Figure 1.7 demonstrates two calls:

• negativecheck(100) — the assertion passes (100 >= 0 is True), so the square, 10000, is printed. The function itself returns None (it has no return statement), which is why the next line printed by the outer print(negativecheck(100)) is None.
• negativecheck(-350) — the assertion fails (-350 >= 0 is False), so AssertionError: OOPS... Negative Number is raised and the print(number*number) line inside the function never runs. Since this second call's exception is never caught by any except block, it propagates all the way up and the program terminates with a traceback.