A unit test checks that one small piece of code, usually a single function, behaves correctly given a known input, and automated tests let a team change code confidently because any broken behavior is caught immediately rather than discovered by a user in production. You will learn to write simple tests that assert an expected output matches the actual output, and the basics of a debugger: a tool that lets you pause code mid-execution and inspect the exact value of every variable at that moment, which is almost always faster than scattering print statements everywhere. You will also learn a systematic debugging process: reproduce the bug reliably, isolate the smallest case that still shows it, form a hypothesis about the cause, and test that hypothesis directly, rather than guessing randomly.