This is very doable, and the API question you're unsure about is actually the solved part. Here's the landscape.
Controlling the KP400
You have three realistic options, in order of how well they fit:
1. Direct local control via python-kasa (recommended). There's a mature, actively maintained open-source Python library, python-kasa, that speaks the Kasa protocol directly to the switches over your LAN — no cloud, no TP-Link account required for the older protocol. The KP400 is on its supported devices list, and since it's a dual-outlet device the library exposes each outlet ("child") individually. It also has a CLI, so you can sanity-check before writing any code:
pip install python-kasa
kasa discover # finds your KP400s on the LAN
kasa --host 192.168.x.x on # or control individual child outlets
One caveat: TP-Link has been migrating newer firmware to an authenticated protocol called KLAP, and there are recent reports of 2025/2026 firmware on some models breaking or removing local control. python-kasa supports KLAP (you just pass your Kasa account credentials), and the KP400 is an older-generation device that most likely still uses the legacy protocol — but this is worth a 5-minute test with kasa discover before committing to the approach. The flip side of this risk: avoid accepting firmware updates once it's working.
2. Cloud control. TP-Link has no official public API, but there's a reverse-engineered tplink-cloud-api library that goes through their cloud with your Kasa credentials. Useful if the controlling program can't be on the same network as the switches, but it's more fragile (unofficial, TP-Link can change it anytime).
3. Home Assistant. If you ever want more home automation, Home Assistant has a first-class TP-Link integration (built on python-kasa) plus weather integrations, and you'd express the logic as an automation rule rather than writing a program. Heavier footprint, but battle-tested. There are also purpose-built hardware controllers for heat cables (e.g., EasyHeat/ETI snow sensors that detect temperature + moisture at the roof), which are the "buy vs. build" alternative — more reliable but less flexible and not cheap.
Weather data
For a US location this is easy and free: the National Weather Service API (api.weather.gov) requires no API key and gives current observations plus hourly forecasts. Open-Meteo is another free, keyless option with a clean JSON API that includes snowfall and freezing-level data. Both are fine for this.
The icing logic (the actual interesting part)
Ice dams form from melt–refreeze cycles: snow on the roof, heat loss melting the underside, water refreezing at the cold eaves. So the trigger condition isn't just "it's freezing" — a reasonable rule is something like:
- On when temperature is in roughly the 15–35°F band and there's snow (recent/forecast snowfall, or snow-depth state you track), since that's the melt-refreeze danger zone.
- Off when it's warm enough that meltwater drains freely (above ~40°F), or so cold that nothing is melting (heat cables do little good at -10°F and burn a lot of power).
- Hysteresis so the wires don't cycle rapidly around a threshold — e.g., separate on/off temperatures and a minimum hold time.
One design decision worth thinking about early: failure behavior. If the weather API is down or the program crashes mid-storm, do you want the wires left in their last state, defaulted on during winter months, or an alert to your phone so you can use the Kasa app manually? Since the cost of a false "off" is roof damage and the cost of a false "on" is an electric bill, I'd bias toward on-when-uncertain during the winter season, plus a notification path.
Suggested shape
A small Python program on any always-on machine on the same LAN as the switches (a Raspberry Pi is the classic choice, but an existing server/NAS works): every 15–30 minutes, fetch weather → evaluate the rule with hysteresis → set switch state via python-kasa → log what it did and why. Give the KP400s DHCP reservations so their IPs don't move. That's maybe 150 lines of code, and it degrades gracefully — the Kasa app keeps working alongside it for manual override.
Given the repo we're sitting in is named heatwires, I suspect you had this in mind. The first concrete step I'd take is the kasa discover test to confirm your specific KP400s' firmware still talks the local protocol — everything else follows from that. Happy to build it out when you're ready.
Sources: python-kasa GitHub, python-kasa supported devices, python-kasa CLI docs, tplink-cloud-api, KLAP local-control issue reports