An introduction to preconditions, postconditions and invariants.

Preconditions, Postconditions, Invariants –- How They Help Write Robust Programs

by Andrzej Krzemieński

From the video description:

Preconditions, postconditions, and invariants are part of what is often called Design by Contract, or Contract Programming. Even if you haven't heard of them, you are probably using them already. In this talk, we are going to see what they are, and how people and tools can make use of them.