reports: Add Balance subtraction methods.

conservancy_beancount/reports/core.py

@@ -133,6 +133,9 @@ class Balance(Mapping[str, data.Amount]):
         self._add_other(retval_map, other)
         return type(self)(retval_map.values())
 
+    def __sub__(self: BalanceType, other: Union[data.Amount, 'Balance']) -> BalanceType:
+        return self.__add__(-other)
+
     def __eq__(self, other: Any) -> bool:
         if isinstance(other, Balance):
             clean_self = self.clean_copy()
@@ -228,6 +231,10 @@ class MutableBalance(Balance):
         self._add_other(self._currency_map, other)
         return self
 
+    def __isub__(self: BalanceType, other: Union[data.Amount, Balance]) -> BalanceType:
+        self._add_other(self._currency_map, -other)
+        return self
+
 
 class RelatedPostings(Sequence[data.Posting]):
     """Collect and query related postings

tests/test_reports_balance.py

@@ -270,6 +270,44 @@ def test_iadd_amount(number, currency):
         assert balance['USD'] == testutil.Amount(500)
         assert balance[currency] == add_amount
 
+@pytest.mark.parametrize('number,currency', {
+    (50, 'USD'),
+    (-50, 'USD'),
+    (50000, 'BRL'),
+    (-4000, 'BRL'),
+})
+def test_sub_amount(number, currency):
+    start_amount = testutil.Amount(500, 'USD')
+    start_bal = core.Balance([start_amount])
+    sub_amount = testutil.Amount(number, currency)
+    actual = start_bal - sub_amount
+    if currency == 'USD':
+        assert len(actual) == 1
+        assert actual['USD'] == testutil.Amount(500 - number)
+    else:
+        assert len(actual) == 2
+        assert actual['USD'] == start_amount
+        assert actual[currency] == -sub_amount
+    assert start_bal == {'USD': start_amount}
+
+@pytest.mark.parametrize('number,currency', {
+    (50, 'USD'),
+    (-50, 'USD'),
+    (50000, 'BRL'),
+    (-4000, 'BRL'),
+})
+def test_isub_amount(number, currency):
+    balance = core.MutableBalance([testutil.Amount(500, 'USD')])
+    sub_amount = testutil.Amount(number, currency)
+    balance -= sub_amount
+    if currency == 'USD':
+        assert len(balance) == 1
+        assert balance['USD'] == testutil.Amount(500 - number)
+    else:
+        assert len(balance) == 2
+        assert balance['USD'] == testutil.Amount(500)
+        assert balance[currency] == -sub_amount
+
 @pytest.mark.parametrize('mapping', [
     {},
     {'USD': 0},