Python comes with a handy-dandy assert keyword that you can use for simple sanity checks. An assertion is simply a boolean expression that checks if its conditions return true or not. If the assertion is true, the program continues. If it’s false, it throws an AssertionError, and your program will stop. Assertions are also a great debugging tool, as they can stop your program in exactly the spot where an error has occurred. This is great for rooting out unreliable data or faulty assumptions.

To make an assertion, just use the assert keyword followed by a condition that should be True:

>>> input_value = 25
>>> assert input_value > 0

If our assertion fails, however:

>>> input_value = 25
>>> assert input_value > 100
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>

assert Is For Sanity Checks, Not For Production!

Assertions are great for simple self-checks and sanity tests. You shouldn’t, however, use assertions for failure cases that can occur because of bad user input or operating system/environment failures, such as a file not being found. These situations are much better suited to an exception or an error message.

Assertions can be disabled at run time, by starting the program with python -O, so you shouldn’t rely on assertions to run in production code. Don’t use them for validation!