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jbranchaud
41 lines
1.7 KiB
Markdown
41 lines
1.7 KiB
Markdown
# Control Passing Of Time In Tests
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While it is nice to be able to write pure functional code, our software still
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lives in the real world and may have to relate to or depend on the passing of
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time. In order to test this kind of code, we need time to behave in a reliable,
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deterministic way. One of the best ways to create a testing environment where
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that is true is to bring in tooling that hijacks time.
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The [`freezegun` module](https://github.com/spulec/freezegun) is a great tool
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for that job. We can use it to freeze time at a specific testable point, advance
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time a specific amount, and much more.
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Here is an example from the tests for [my CLI-based time tracking
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app](https://github.com/jbranchaud/py-vmt/blob/acb26e4840279d936a12f16c505ca7e75e9a6d20/tests/src/py_vmt/test_cli.py#L21)
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where I freeze time before starting a session. That gives me a chance to assert
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about the exact start time that is output by the command. Then I can advance
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time a little and assert that the `status` command outputs the correct thing.
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```python
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import datetime
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from freezegun import freeze_time
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# some other test setup omitted ...
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initial_datetime = datetime.datetime(
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2026, 3, 14, 15, 5, 11, 0, datetime.timezone.utc
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)
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with freeze_time(initial_datetime) as frozen_datetime:
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# start a session
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start_result = runner.invoke(cli, ["start", "my-project"])
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output = "Started tracking 'my-project' at 10:05AM"
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assert output in start_result.output
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frozen_datetime.tick(delta=datetime.timedelta(minutes=30))
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# check status
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status_result = runner.invoke(cli, ["status"])
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output = "Tracking 'my-project' for 30m (since 10:05AM)"
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assert output in status_result.output
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```
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