Disable default constructor for specialized Quantity types

Specialized quantities can only be instantiated through custom
constructors after this.

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms

src/frequenz/sdk/timeseries/_quantities.py

@@ -7,7 +7,7 @@
 
 import math
 from datetime import timedelta
-from typing import Self, TypeVar, overload
+from typing import Any, NoReturn, Self, TypeVar, overload
 
 QuantityT = TypeVar("QuantityT", "Quantity", "Power", "Current", "Voltage", "Energy")
 
@@ -279,8 +279,29 @@ def __eq__(self, other: object) -> bool:
         return self._base_value == other._base_value
 
 
+class _NoDefaultConstructible(type):
+    """A metaclass that disables the default constructor."""
+
+    def __call__(cls, *_args: Any, **_kwargs: Any) -> NoReturn:
+        """Raise a TypeError when the default constructor is called.
+
+        Args:
+            _args: ignored positional arguments.
+            _kwargs: ignored keyword arguments.
+
+        Raises:
+            TypeError: Always.
+        """
+        raise TypeError(
+            "Use of default constructor NOT allowed for "
+            f"{cls.__module__}.{cls.__qualname__}, "
+            f"use one of the `{cls.__name__}.from_*()` methods instead."
+        )
+
+
 class Power(
     Quantity,
+    metaclass=_NoDefaultConstructible,
     exponent_unit_map={
         -3: "mW",
         0: "W",
@@ -422,6 +443,7 @@ def __truediv__(self, other: Current | Voltage) -> Voltage | Current:
 
 class Current(
     Quantity,
+    metaclass=_NoDefaultConstructible,
     exponent_unit_map={
         -3: "mA",
         0: "A",
@@ -485,7 +507,11 @@ def __mul__(self, voltage: Voltage) -> Power:
         return Power.from_watts(self._base_value * voltage._base_value)
 
 
-class Voltage(Quantity, exponent_unit_map={0: "V", -3: "mV", 3: "kV"}):
+class Voltage(
+    Quantity,
+    metaclass=_NoDefaultConstructible,
+    exponent_unit_map={0: "V", -3: "mV", 3: "kV"},
+):
     """A voltage quantity."""
 
     @classmethod
@@ -568,6 +594,7 @@ def __mul__(self, current: Current) -> Power:
 
 class Energy(
     Quantity,
+    metaclass=_NoDefaultConstructible,
     exponent_unit_map={
         0: "Wh",
         3: "kWh",

tests/timeseries/test_quantities.py

@@ -177,6 +177,10 @@ def test_power() -> None:
     assert power == Power.from_megawatts(0.0012)
     assert power != Power.from_watts(1000.0)
 
+    with pytest.raises(TypeError):
+        # using the default constructor should raise.
+        Power(1.0, exponent=0)
+
 
 def test_current() -> None:
     """Test the current class."""
@@ -192,6 +196,10 @@ def test_current() -> None:
     assert current == Current.from_amperes(6.0)
     assert current != Current.from_amperes(5.0)
 
+    with pytest.raises(TypeError):
+        # using the default constructor should raise.
+        Current(1.0, exponent=0)
+
 
 def test_voltage() -> None:
     """Test the voltage class."""
@@ -209,6 +217,10 @@ def test_voltage() -> None:
     assert voltage == Voltage.from_volts(6.0)
     assert voltage != Voltage.from_volts(5.0)
 
+    with pytest.raises(TypeError):
+        # using the default constructor should raise.
+        Voltage(1.0, exponent=0)
+
 
 def test_energy() -> None:
     """Test the energy class."""
@@ -225,6 +237,10 @@ def test_energy() -> None:
     assert energy == Energy.from_kilowatt_hours(6.0)
     assert energy != Energy.from_kilowatt_hours(5.0)
 
+    with pytest.raises(TypeError):
+        # using the default constructor should raise.
+        Energy(1.0, exponent=0)
+
 
 def test_quantity_compositions() -> None:
     """Test the composition of quantities."""