Add unary negation.

Author: mjziebarth

README.md

@@ -175,6 +175,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### [Unreleased]
 #### Added
 - Add computation of unit powers in C++ and Python.
+- Add unary negation operator to `Quantity`.
 
 #### Changed
 - Fix array values of `Quantity` with dimension larger than one causing

cyantities/quantity.pyx

@@ -618,6 +618,25 @@ cdef class Quantity:
         return _power_quantity(self, exponent)
 
 
+    def __neg__(self):
+        """
+        Unary negative.
+        """
+        # Mostly a copy, we just have to see which part of the value
+        # (scalar or ndarray?) we have to negate:
+        cdef Quantity res = Quantity.__new__(Quantity)
+        if self._is_scalar:
+            res._cyinit(
+                True, -self._val, None, self._unit
+            )
+        else:
+            res._cyinit(
+                False, dummy_double[0], -self._val_object, self._unit
+            )
+
+        return res
+
+
     def __eq__(self, other):
         # First the case that the other is not a Quantity.
         # This results in nonzero only if this quantity is

testing/test_quantities.py

@@ -76,8 +76,10 @@ def test_quantity():
     assert np.all(q6 == Quantity(np.array([1.0, 2.0, 3.0]) - 1.0, "m"))
     q7 = q0 - q1
     assert np.all(q7 == Quantity(1.0 - np.array([1.0, 2.0, 3.0]), "m"))
+    assert np.all(q7 == -q6)
     with pytest.raises(RuntimeError):
         q0 - q3
+    assert q0 - 2*q0 == -q0
 
     # Test conversion of dimensionless Quantities to NumPy arrays:
     q8 = q1 / Unit('m')