Remove dunder iadd, isub and imul methods for Quantity

Having them mutating the objects in-place is not the best option as
number are assumed to be immutable in Python.  If we make them mutable,
then bad things will happen if they are used as default arguments in
functions, or if we want to provide a zero singleton for example.

We also mention explicitly now that Quantities are immutable.

This partially reverts 1043360 (the
tests are kept).

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/sdk/timeseries/_quantities.py

@@ -24,7 +24,10 @@
 
 
 class Quantity:
-    """A quantity with a unit."""
+    """A quantity with a unit.
+
+    Quantities try to behave like float and are also immutable.
+    """
 
     _base_value: float
     """The value of this quantity in the base unit."""
@@ -249,48 +252,6 @@ def __mul__(self, percent: Percentage) -> Self:
         product._base_value = self._base_value * percent.as_fraction()
         return product
 
-    def __iadd__(self, other: Self) -> Self:
-        """Add another quantity to this one.
-
-        Args:
-            other: The other quantity.
-
-        Returns:
-            This quantity.
-        """
-        if not type(other) is type(self):
-            return NotImplemented
-        self._base_value += other._base_value
-        return self
-
-    def __isub__(self, other: Self) -> Self:
-        """Subtract another quantity from this one.
-
-        Args:
-            other: The other quantity.
-
-        Returns:
-            This quantity.
-        """
-        if not type(other) is type(self):
-            return NotImplemented
-        self._base_value -= other._base_value
-        return self
-
-    def __imul__(self, percent: Percentage) -> Self:
-        """Multiply this quantity by a percentage.
-
-        Args:
-            percent: The percentage.
-
-        Returns:
-            This quantity.
-        """
-        if not isinstance(percent, Percentage):
-            return NotImplemented
-        self._base_value *= percent.as_fraction()
-        return self
-
     def __gt__(self, other: Self) -> bool:
         """Return whether this quantity is greater than another.
 
@@ -413,7 +374,7 @@ class Power(
 ):
     """A power quantity.
 
-    Objects of this type are wrappers around `float` values.
+    Objects of this type are wrappers around `float` values and are immutable.
 
     The constructors accept a single `float` value, the `as_*()` methods return a
     `float` value, and each of the arithmetic operators supported by this type are
@@ -586,7 +547,7 @@ class Current(
 ):
     """A current quantity.
 
-    Objects of this type are wrappers around `float` values.
+    Objects of this type are wrappers around `float` values and are immutable.
 
     The constructors accept a single `float` value, the `as_*()` methods return a
     `float` value, and each of the arithmetic operators supported by this type are
@@ -682,7 +643,7 @@ class Voltage(
 ):
     """A voltage quantity.
 
-    Objects of this type are wrappers around `float` values.
+    Objects of this type are wrappers around `float` values and are immutable.
 
     The constructors accept a single `float` value, the `as_*()` methods return a
     `float` value, and each of the arithmetic operators supported by this type are
@@ -804,7 +765,7 @@ class Energy(
 ):
     """An energy quantity.
 
-    Objects of this type are wrappers around `float` values.
+    Objects of this type are wrappers around `float` values and are immutable.
 
     The constructors accept a single `float` value, the `as_*()` methods return a
     `float` value, and each of the arithmetic operators supported by this type are
@@ -923,7 +884,7 @@ class Frequency(
 ):
     """A frequency quantity.
 
-    Objects of this type are wrappers around `float` values.
+    Objects of this type are wrappers around `float` values and are immutable.
 
     The constructors accept a single `float` value, the `as_*()` methods return a
     `float` value, and each of the arithmetic operators supported by this type are
@@ -1036,7 +997,7 @@ class Percentage(
 ):
     """A percentage quantity.
 
-    Objects of this type are wrappers around `float` values.
+    Objects of this type are wrappers around `float` values and are immutable.
 
     The constructors accept a single `float` value, the `as_*()` methods return a
     `float` value, and each of the arithmetic operators supported by this type are