Module 9

Table of Contents

Debugging

9.2 Assertion

An assertion is a sanity check to make sure your code isn’t doing something obviously wrong. These sanity checks are performed by assert statements. If the sanity check fails, then an AssertionError exception is raised. In code, an assert statement consists of the following:

  • The assert keyword

  • A condition (that is, an expression that evaluates to True or False)

  • A comma

  • A string to display when the condition is False

For example, enter the following into the interactive shell:

>>> assert sum([1, 2, 3]) == 6, "Should be 6"
>>> assert sum([1, 2, 2]) == 6, "Should be 6"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError: Should be 6

In plain English, an assert statement says, “I assert that this condition holds true, and if not, there is a bug somewhere in the program.” Unlike exceptions, your code should not handle assert statements with try and except; if an assert fails, your program should crash.

By failing fast like this, you shorten the time between the original cause of the bug and when you first notice the bug. This will reduce the amount of code you will have to check before finding the code that’s causing the bug.

Assertions are for programmer errors, not user errors. For errors that can be recovered from (such as a file not being found or the user entering invalid data), raise an exception instead of detecting it with an assert statement.

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