An assertion is a condition that we're claiming should be true at this point in the program. Typically, it summarizes the state of the program's variables. Assertions can help explain the relationships among variables, review what has happened so far in the program, and show that if statements and for or while loops have the desired effect.

When a program is correct, all of the assertions are true no matter what inputs are provided. When a program has an error, at least one assertion winds up false for some combination of inputs.

Python directly supports assertions through an assert statement. There are two forms:

If the condition is False, the program is in error; this statement raises an AssertionError exception. If the condition is True, the program is correct, this statement does nothing more.

If the second form of the statement is used, and an expression is given, an exception is raised using the value of the expression. We'll cover exceptions in detail in Chapter 17, Exceptions. If the expression is a string, it becomes an the value associated with the AssertionError exception.

Here's a typical example:

max= 0
if a < b: max= b
if b < a: max= a
assert (max == a or max == b) and max >= a and max >= b

If the assertion condition is true, the program continues. If the assertion condition is false, the program raises an AssertionError exception and stops, showing the line where the problem was found.

Run this program with a equal to b and not equal to zero; it will raise the AssertionError exception. Clearly, the if statements don't set max to the largest of a and b when a = b. There is a problem in the if statements, and the presence of the problem is revealed by the assertion.