| # Snapshot Testing | |
| ## What is snapshot testing? | |
| Some tests that run for Textual are snapshot tests. | |
| When you first run a snapshot test, a screenshot of an app is taken and saved to disk. | |
| Next time you run it, another screenshot is taken and compared with the original one. | |
| If the screenshots don't match, it means something has changed. | |
| It's up to you to tell the test system whether that change is expected or not. | |
| This allows us to easily catch regressions in how Textual outputs to the terminal. | |
| Snapshot tests run alongside normal unit tests. | |
| ## How do I write a snapshot test? | |
| 1. Inject the `snap_compare` fixture into your test. | |
| 2. Pass in the path to the file which contains the Textual app. | |
| ```python | |
| def test_grid_layout_basic_overflow(snap_compare): | |
| assert snap_compare("docs/examples/guide/layout/grid_layout2.py") | |
| ``` | |
| `snap_compare` can take additional arguments such as `press`, which allows | |
| you to simulate key presses etc. | |
| See the signature of `snap_compare` for more info. | |
| ## A snapshot test failed, what do I do? | |
| When a snapshot test fails, a report will be created on your machine, and you | |
| can use this report to visually compare the output from your test with the historical output for that test. | |
| This report will be visible at the bottom of the terminal after the `pytest` session completes, | |
| or, if running in CI, it will be available as an artifact attached to the GitHub Actions summary. | |
| If you're happy that the new output of the app is correct, you can run `pytest` with the | |
| `--snapshot-update` flag. This flag will update the snapshots for any test that is executed in the run, | |
| so to update a snapshot for a single test, run only that test. | |
| With your snapshot on disk updated to match the new output, running the test again should result in a pass. | |