Support negation of quantities (#515)

Author: Marenz

src/frequenz/sdk/timeseries/_quantities.py

@@ -287,6 +287,16 @@ def __eq__(self, other: object) -> bool:
         assert isinstance(other, self.__class__)
         return self._base_value == other._base_value
 
+    def __neg__(self) -> Self:
+        """Return the negation of this quantity.
+
+        Returns:
+            The negation of this quantity.
+        """
+        negation = type(self).__new__(type(self))
+        negation._base_value = -self._base_value
+        return negation
+
 
 class _NoDefaultConstructible(type):
     """A metaclass that disables the default constructor."""

tests/timeseries/test_quantities.py

@@ -293,3 +293,29 @@ def test_percentage() -> None:
     assert f"{pct}" == "20.4 %"
     assert pct.as_percent() == 20.4
     assert pct.as_fraction() == 0.204
+
+
+def test_neg() -> None:
+    """Test the negation of quantities."""
+    power = Power.from_watts(1000.0)
+    assert -power == Power.from_watts(-1000.0)
+    assert -(-power) == power
+
+    voltage = Voltage.from_volts(230.0)
+    assert -voltage == Voltage.from_volts(-230.0)
+    assert -(-voltage) == voltage
+
+    current = Current.from_amperes(2)
+    assert -current == Current.from_amperes(-2)
+    assert -(-current) == current
+
+    energy = Energy.from_kilowatt_hours(6.2)
+    assert -energy == Energy.from_kilowatt_hours(-6.2)
+
+    freq = Frequency.from_hertz(50)
+    assert -freq == Frequency.from_hertz(-50)
+    assert -(-freq) == freq
+
+    pct = Percentage.from_fraction(30)
+    assert -pct == Percentage.from_fraction(-30)
+    assert -(-pct) == pct