# Testing a Viewer

A plot is an ordinary function or object: it takes the run's data and returns
drawables (see [Plots and Drawables](../viewing/plots.md)). So it can be tested without a
window, a screen or the `gui` extra, by giving it the data of a stored run and
checking what it returns.

```python
# tests/test_viz.py
from plesty.lib.monitor import Line, RunData

from power_viz import power      # the plot function of the viewer being tested


def test_power_draws_one_point_per_step():
    data = RunData("tests/runs/powerscan_20260925-101534")
    data.poll()                  # read the rows saved so far

    drawing = power(data)

    assert len(drawing) == 1
    assert isinstance(drawing[0], Line)
    assert list(drawing[0].x) == [500.0, 550.0, 600.0, 650.0, 700.0]
```

- `RunData(run_folder)` reads nothing yet; `poll()` reads every row saved so far and
  returns how many it read. In a viewer, `Viz` does this at each refresh.
- `@viz.plot` returns the function unchanged, so the test calls `power(data)`
  directly. For a `Plot` object, call `plot.draw(data)`.
- Keep a small finished run in the module's tests, made by running the experiment
  once on its simulated devices, so the test does not depend on a lab computer.

A viewer's `reads=` function can be tested the same way: call it with a row and a
`Run`, and check the fields it returns.