Improve unit conversion for sensor entities

[emontnemery]

Proposal:
Move temperature unit conversion from Entity._async_write_ha_state to sensor base class SensorEntity.

Suggested implementation alternatives:

• Add new properties converted_state and converted_unit_of_measurement to Entity with default implementations which simply returns self.state, self.unit_of_measurement, override those in SensorEntity to do unit conversion based on device class and update Entity._async_write_ha_state to read from self.converted_state instead of self.state and from converted_unit_of_measurement instead of unit_of_measurement.
  Pro:

  • Very simple to implement

  Con:

  • Entity has more sensor specific support, not less, after this change

• Make SensorEntity.state property handle unit conversions and update all sensors to no longer override the state and unit_of_measurement properties, but instead override two new properties named sensor_value and sensor_unit_of_measurement, or similar, which represent the values read from the sensor.
  Pro:

  • No sensor specific support in Entity after this change
  • Reasonably simple to implement: It should be possible to simply rename all sensors' implementations of state and unit_of_measurement without any manual changes

In both cases:

• Backwards compatibility for custom components is achieved by falling back to the old Entity._async_write_ha_state during a transition period.
• Integrations can override the default behavior

Other considerations:
climate, water_heater, weather also do unit conversion, but it's already done in a clean way in the base components

[MartinHjelmare]

What will be the criteria for falling back to the old behavior for custom integrations?

[emontnemery]

We can keep the old conversion unchanged, and print a warning if it's executed.
If it's desirable to allow a sensor to report temperature in a custom unit, we could do the conversion only if the entity doesn't have sensor_unit_of_measurement.

Edit:
In addition to sensor, air_quality and geo_location also use unit_of_measurement. It's also the case for a handful of other entities.

[MartinHjelmare]

I'm for alternative number 2.

[emontnemery]

PR implementing alternative 2: home-assistant/core#48261