Python Integration Testing Patterns — ELI5

Imagine you’re building a puzzle. You check each piece individually — they all look fine, no bent edges, colors are right. But when you try to connect them, some pieces don’t fit together. Each piece was perfect on its own, but together they fail.

Integration testing is checking that the pieces fit together.

In software, different parts of your program work independently. The login system checks passwords. The database stores user information. The email system sends welcome messages. Each part might work perfectly by itself. But when the login system tries to save a user to the database and trigger a welcome email — does that whole chain work?

Unit tests check individual pieces. Integration tests check that pieces work together. They’re testing the handshake between parts.

Think of it like checking that your TV remote actually controls your TV. You could test the remote separately (batteries work, buttons click) and the TV separately (screen turns on, speakers work). But the real question is: when you press the remote button, does the TV respond?

Integration tests are usually slower than unit tests because they involve real connections — actual databases, actual files, actual network calls. But they catch a class of bugs that unit tests simply can’t: the bugs that live in the gaps between components.

One thing to remember: Integration tests verify the connections between parts — the wires, the handshakes, the data flowing from one component to the next.