Reject a time based screensaver without both times

[joostlek]

Proposed Changes

(Describe the changes and rationale behind them)

Setting the screensaver time did not work. The device wants both start_time and end_time on every time_based write, even one that only toggles enabled, and it fails three different ways without them:

mode_params	Device response
start_time alone	200, and quietly ignored
end_time alone	400 Bad end time params
enabled alone, no times	400 Bad end time params

The first row is the damaging one, and the likely cause of the report. Changing only the start time — the obvious way to use this — reported success, and the returned Display still held the old times, so nothing indicated the write had been dropped:

# before: reported success, changed nothing
await lametric.display(
    screensaver_mode=ScreensaverMode.TIME_BASED,
    screensaver_start_time=time(21, 0),
)

display() now raises ValueError for the time based mode unless both times are given, so the mismatch surfaces at the call rather than looking like it worked. The other modes take no times and are unaffected, which a test covers.

Requiring both times even to disable the mode is the device's rule, not a choice made here. Callers that only want to toggle it can pass the times they read back from display(), or switch modes instead, since enabling one mode disables the other.

Verified against a LaMetric TIME (model sa8, OS 3.2.7): all three shapes above now raise before any request is sent, and a write with both times applies and reads back correctly.

Follow-up worth considering

The 400 cases surfaced as LaMetricError: Error occurred while connecting to the LaMetric device at <host>, which is actively misleading, since nothing was wrong with the connection. _request() discards the response body on ClientResponseError, so the device's own Bad end time params never reaches the caller. Including it would have made this far easier to diagnose, but it changes exception messages across every endpoint, so it belongs in its own change.

Related Issues

(Github link to related issues or pull requests)

None.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The focused validation matches documented device behavior and is adequately tested.

Pull request overview

Adds early validation for device-required time-based screensaver parameters.

Changes:

• Requires both start and end times for time-based writes.
• Tests invalid combinations and unaffected modes.

File summaries

File	Description
src/demetriek/device.py	Validates time-based screensaver writes.
tests/test_display.py	Covers validation and other modes.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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