What is the use of "assert" in Python?

Question

I have been reading some source code and in several places I have seen the usage of `assert`. What does it mean exactly? What is its usage?

Answer 1

The assert statement is used as a way to test if your code is getting results that it should not be getting. All it does is check what is provided for it, and if True, nothing happens, but if False, it throws some kind of exception. This exception defaults to AssertionError, but can be changed in the assertion itself.

An example of a default assertion that will throw AssertionError:

assert False

An example of an assertion that will throw a custom error

assert False, "Function1 is getting NoneType"

This has a variety of uses from testing if functions are receiving/producing the correct results in test cases, or tracking down the root of an error.

So to answer your question directly, the assert statement doesn't "do" any thing for the program itself, at least in the same way that an if then statement compares something, but it does make for a useful testing tool allowing for easier automatic testing.