Testing code that depends on time and randomness

Goal

Make behaviour that depends on "now" or on random draws testable with exact expectations, and remove a common source of flaky tests.

Prerequisites

Code paths that read the clock or draw random values.

Steps

1. Pass the time source as a dependency: a function now() or a small Clock object, defaulting to the real clock in production.
2. Pass the random source likewise (random.Random(seed) or secrets), never import-level global state.
3. In tests, supply a fake clock that returns fixed instants and advances only when the test says so; supply a seeded generator.
4. Where injection is impossible (third-party code), patch narrowly with monkeypatch.setattr or unittest.mock.patch at the point of use, for the duration of one test.
5. Assert on exact values (expires_at == start + 3600), not on ranges.
6. Test boundaries: midnight, month ends, daylight-saving transitions, leap days.

Expected result

Tests for expiry, scheduling and backoff run instantly and deterministically; the production code has no test-specific branches.

Limits and test basis

Patching module attributes affects all code using that module during the patch; keep patches local. Sleeping in tests to "wait for time to pass" is the anti-pattern this procedure removes. Mechanics follow the cited documentation.