Fix quantity rounding formatting bug

Certain numbers would be rendered wrongly due to rounding in the python formatter, e.g. Voltage.from_volts(999.9999850988388) would render "1000 V".
This results in a faulty round-trip conversion (`num -> str -> num` no longer matching).

Signed-off-by: Mathias L. Baumann <[email protected]>

Author: Marenz

src/frequenz/sdk/timeseries/_quantities.py

@@ -199,6 +199,7 @@ def __str__(self) -> str:
         """
         return self.__format__("")
 
+    # pylint: disable=too-many-branches
     def __format__(self, __format_spec: str) -> str:
         """Return a formatted string representation of this quantity.
 
@@ -250,8 +251,22 @@ def __format__(self, __format_spec: str) -> str:
         if math.isinf(self._base_value) or math.isnan(self._base_value):
             return f"{self._base_value} {self._exponent_unit_map[0]}"
 
-        abs_value = abs(self._base_value)
-        exponent = math.floor(math.log10(abs_value)) if abs_value else 0
+        if abs_value := abs(self._base_value):
+            precision_pow = 10 ** (precision)
+            # Prevent numbers like 999.999999 being rendered as 1000 V
+            # instead of 1 kV.
+            # This could happen because the str formatting function does
+            # rounding as well.
+            # This is an imperfect solution that works for _most_ cases.
+            # isclose parameters were chosen according to the observed cases
+            if math.isclose(abs_value, precision_pow, abs_tol=1e-4, rel_tol=0.01):
+                # If the value is close to the precision, round it
+                exponent = math.ceil(math.log10(precision_pow))
+            else:
+                exponent = math.floor(math.log10(abs_value))
+        else:
+            exponent = 0
+
         unit_place = exponent - exponent % 3
         if unit_place < min(self._exponent_unit_map):
             unit = self._exponent_unit_map[min(self._exponent_unit_map.keys())]

tests/timeseries/test_quantities.py

@@ -107,6 +107,7 @@ def test_string_representation() -> None:
     assert (
         repr(Quantity(1.024445, exponent=0)) == "Quantity(value=1.024445, exponent=0)"
     )
+    assert f"{Quantity(0.50001, exponent=0):.0}" == "1"
     assert f"{Quantity(1.024445, exponent=0)}" == "1.024"
     assert f"{Quantity(1.024445, exponent=0):.0}" == "1"
     assert f"{Quantity(0.124445, exponent=0):.0}" == "0"
@@ -154,6 +155,7 @@ def test_string_representation() -> None:
     assert f"{Energy.from_watt_hours(0.124445):.0}" == "0 Wh"
     assert f"{Power.from_watts(-0.0):.0}" == "-0 W"
     assert f"{Power.from_watts(0.0):.0}" == "0 W"
+    assert f"{Voltage.from_volts(999.9999850988388)}" == "1 kV"
 
 
 def test_isclose() -> None: