reports: Add Balance.__neg__() method.

conservancy_beancount/reports/core.py

@@ -67,6 +67,11 @@ class Balance(Mapping[str, data.Amount]):
         amounts.sort(key=lambda amt: abs(amt.number), reverse=True)
         return ', '.join(str(amount) for amount in amounts)
 
+    def __neg__(self) -> 'Balance':
+        return type(self)(
+            (key, -amt) for key, amt in self.items()
+        )
+
     def __getitem__(self, key: str) -> data.Amount:
         return data.Amount(self._currency_map[key], key)
 

tests/test_reports_balance.py

@@ -67,6 +67,22 @@ def test_mixed_balance():
     assert not balance.is_zero()
     assert all(balance[key] == amt for key, amt in amounts.items())
 
+@pytest.mark.parametrize('balance_map_kwargs', [
+    {},
+    {'USD': 0},
+    {'EUR': 10},
+    {'JPY': 20, 'BRL': 30},
+    {'EUR': -15},
+    {'JPY': -25, 'BRL': -35},
+    {'JPY': 40, 'USD': 0, 'EUR': -50},
+])
+def test_neg(balance_map_kwargs):
+    amounts = testutil.balance_map(**balance_map_kwargs)
+    actual = -core.Balance(amounts.items())
+    assert set(actual) == set(balance_map_kwargs)
+    for key in balance_map_kwargs:
+        assert actual[key] == -amounts[key]
+
 @pytest.mark.parametrize('balance_map_kwargs,expected', [
     ({}, "Zero balance"),
     ({'JPY': 0, 'BRL': 0}, "Zero balance"),