Skip to main content

UnitSystem dataclass is now frozen

· One min read
Shay Levy

UnitSystem class is changed to a frozen data class, and all instances derived from it are now frozen.

The following unit systems that derived from the UnitSystem class are now frozen:

  • METRIC_SYSTEM
  • US_CUSTOMARY_SYSTEM

The reason for this change is that Unit systems are constants that should not be modified. An integration that modifies these constants can break unit conversions and create undesired output for other components of Home Assistant.

With a frozen data class an attempt to modify the UnitSystem constant will fail:

dataclasses.FrozenInstanceError: cannot assign to field 'temperature_unit'

This change was introduced in the home assistant core PR #140954.