Pass the dependency in
How do you swap a real dependency for a double? The simplest technique is dependency injection, a fancy name for something you already know from Advanced Python: functions are values, so pass the dependency in as a parameter.
def temperature_label(fetch_temp, city): t = fetch_temp(city) ...
In production, callers hand in the real network function. In tests, you hand in a stub that returns a canned number instantly. The unit cannot tell the difference, and your test becomes fast and deterministic.
This is why testable code and well-designed code are usually the same code: a function that receives its dependencies is also easier to reuse and reason about.
Code exercise · python
Run this. stub_fetch answers from a hard-coded dictionary, so the three tests run instantly with zero network and always get the same temperatures.
Quiz
Why not simply test temperature_label against the real weather API?
Code exercise · python
Your turn. greeting reads the hour from an injected now_hour function. Write stubs (lambda: 9 is enough) to test all five cases, including the boundary probes at exactly 12 and 18 that lesson 3-2 taught you to include. Then print: 5 tests passed without waiting for the clock
Code exercise · python
One more injection rep, solo. shipping_label asks an injected get_country function where an order ships. Stub it twice with lambdas — one returning "US", one returning "PE" — to test both branches, then print: 2 tests passed without a database