“Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.” – Brian W. Kernighan.
Everyone tests their software to some extent, if only by running it and trying it out (technically known as “smoke testing”). Most programmers do a certain amount of exploratory testing, which involves running through various functional paths in your code and seeing if they work.
Systematic testing, however, is a different matter. Systematic testing simply cannot be done properly without a certain (large!) amount of automation, because every change to the software means that the software needs to be tested all over again.
This is an introduction to some lower level automated testing concepts, and how to use built-in Python constructs to start writing tests.