# Tests and screenshots

> Test Pyzed apps headlessly: render your Python GUI without a window for unit tests, documentation screenshots and continuous integration.

Source: https://albakore.github.io/pyzed/guides/testing/

`app.screenshot(path)` renders the app's current state **off-screen** and saves a PNG.
It needs no window, display or GPU (it uses the software renderer), so it works in CI.

```python
app = MyApp()
app.items = ["one", "two"]          # set up the state you want to show
app.screenshot("docs/list.png", size=(480, 320), scale=2)
```

- `size`: logical size; defaults to `App.size`.
- `scale`: PNG pixel density (2 = retina).
- `wait`: seconds to wait before capturing, in case `on_start` started some work.

Every image in this documentation is generated this way:

## Testing the logic

Since the state lives in your object, `update` and callbacks are tested like plain Python:

```python
def test_counter():
    app = Counter()
    app.update("increment")
    app.update("increment")
    assert app.count == 2
```

## Testing that the view renders

`view()` must return a `Widget` without raising. A screenshot makes a good smoke test:

```python
def test_view_renders(tmp_path):
    app = MyApp()
    app.screenshot(tmp_path / "ui.png", size=(800, 600))
    assert (tmp_path / "ui.png").stat().st_size > 0
```

If `view()` raises, the screenshot shows the traceback instead of the UI, just like the window.