A fake and a mock stand in for an LLM client
No network and no API key: the Summariser only needs an object with a .complete(prompt) method. Change the fake's answer and see the result change.
A mock or fake is a stand-in for something slow, unsafe or unpredictable (the network, the clock, a database, an AI model), so a test checks only your own code. unittest.mock provides Mock, MagicMock and patch for this.
Change the code and press Run (Ctrl+Enter). Predict what will happen first, then break it on purpose and read what changes. Your edits are saved and match the lesson page.
Ready for more? This phase has 6 hands-on exercises with automatic checks.
Practice questions
Write the code in the editor, run it, then open the model answer to compare.
Write a FakeClock class with a now() method that returns a time you set, and a function greeting(clock) that returns "Good morning" before 12 and "Good afternoon" otherwise. Test both cases.
Use Mock(side_effect=[...]) to test a fetch_with_retry(client) function that retries once if the first call raises ConnectionError.
Use patch.object to replace a method on a class for one with block and show that the original returns afterwards.
Explain it without notes
What is the difference between a stub, a fake and a mock?
Where should you patch: where a name is defined or where it is used? Explain with an example.
How do you test a function that calls an AI model without calling the model?
What is over-mocking and how do you avoid it?
What does create_autospec give you that a plain Mock does not?
Expected output
fake result: 'Tests protect your code.'
fake saw: ['Summarise in one sentence: Long essay about tests']
mock result: Short.
prompt check passed
error path: model did not answer