Cache sensor precision calculation (#140019)

homeassistant/components/sensor/__init__.py

@@ -675,22 +675,13 @@ class SensorEntity(Entity, cached_properties=CACHED_PROPERTIES_WITH_ATTR_):
             ):
                 # Deduce the precision by finding the decimal point, if any
                 value_s = str(value)
-                precision = (
-                    len(value_s) - value_s.index(".") - 1 if "." in value_s else 0
-                )
-
                 # Scale the precision when converting to a larger unit
                 # For example 1.1 Wh should be rendered as 0.0011 kWh, not 0.0 kWh
-                ratio_log = max(
+                precision = (
-                    0,
+                    len(value_s) - value_s.index(".") - 1 if "." in value_s else 0
-                    log10(
+                ) + converter.get_unit_floored_log_ratio(
-                        converter.get_unit_ratio(
                     native_unit_of_measurement, unit_of_measurement
                 )
-                    ),
-                )
-                precision = precision + floor(ratio_log)
-
                 value = f"{converted_numerical_value:z.{precision}f}"
             else:
                 value = converted_numerical_value

homeassistant/util/unit_conversion.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 from collections.abc import Callable
 from functools import lru_cache
+from math import floor, log10
 
 from homeassistant.const import (
     CONCENTRATION_PARTS_PER_BILLION,
@@ -144,6 +145,15 @@ class BaseUnitConverter:
         from_ratio, to_ratio = cls._get_from_to_ratio(from_unit, to_unit)
         return from_ratio / to_ratio
 
+    @classmethod
+    @lru_cache
+    def get_unit_floored_log_ratio(
+        cls, from_unit: str | None, to_unit: str | None
+    ) -> float:
+        """Get floored base10 log ratio between units of measurement."""
+        from_ratio, to_ratio = cls._get_from_to_ratio(from_unit, to_unit)
+        return floor(max(0, log10(from_ratio / to_ratio)))
+
     @classmethod
     @lru_cache
     def _are_unit_inverses(cls, from_unit: str | None, to_unit: str | None) -> bool:

tests/util/test_unit_conversion.py

@@ -902,8 +902,8 @@ def test_convert_nonnumeric_value(
     ("converter", "from_unit", "to_unit", "expected"),
     [
         # Process all items in _GET_UNIT_RATIO
-        (converter, item[0], item[1], item[2])
+        (converter, from_unit, to_unit, expected)
-        for converter, item in _GET_UNIT_RATIO.items()
+        for converter, (from_unit, to_unit, expected) in _GET_UNIT_RATIO.items()
     ],
 )
 def test_get_unit_ratio(
@@ -915,13 +915,34 @@ def test_get_unit_ratio(
     assert converter.get_unit_ratio(to_unit, from_unit) == pytest.approx(1 / ratio)
 
 
+@pytest.mark.parametrize(
+    ("converter", "from_unit", "to_unit", "expected"),
+    [
+        # Process all items in _GET_UNIT_RATIO
+        (converter, from_unit, to_unit, expected)
+        for converter, (from_unit, to_unit, expected) in _GET_UNIT_RATIO.items()
+    ],
+)
+def get_unit_floored_log_ratio(
+    converter: type[BaseUnitConverter], from_unit: str, to_unit: str, expected: float
+) -> None:
+    """Test floored log unit ratio.
+
+    Should not use pytest.approx since we are checking these
+    values are exact.
+    """
+    ratio = converter.get_unit_floored_log_ratio(from_unit, to_unit)
+    assert ratio == expected
+    assert converter.get_unit_floored_log_ratio(to_unit, from_unit) == 1 / ratio
+
+
 @pytest.mark.parametrize(
     ("converter", "value", "from_unit", "expected", "to_unit"),
     [
         # Process all items in _CONVERTED_VALUE
-        (converter, list_item[0], list_item[1], list_item[2], list_item[3])
+        (converter, value, from_unit, expected, to_unit)
         for converter, item in _CONVERTED_VALUE.items()
-        for list_item in item
+        for value, from_unit, expected, to_unit in item
     ],
 )
 def test_unit_conversion(