reports: Add Balance.__pos__() method.

I did this while I was working on normalize_amount_func.
It turns out it's not immediately needed, but it's still nice to have.

conservancy_beancount/reports/core.py

@@ -140,6 +140,9 @@ class Balance(Mapping[str, data.Amount]):
     def __neg__(self: BalanceType) -> BalanceType:
         return type(self)(-amt for amt in self.values())
 
+    def __pos__(self: BalanceType) -> BalanceType:
+        return self
+
     def __getitem__(self, key: str) -> data.Amount:
         return self._currency_map[key]
 

tests/test_reports_balance.py

@@ -168,6 +168,20 @@ def test_neg(mapping):
     for key, number in mapping.items():
         assert actual[key] == testutil.Amount(-number, key)
 
+@pytest.mark.parametrize('mapping', [
+    {},
+    {'USD': 0},
+    {'EUR': 10},
+    {'JPY': 20, 'BRL': 30},
+    {'EUR': -15},
+    {'JPY': -25, 'BRL': -35},
+    {'JPY': 40, 'USD': 0, 'EUR': -50},
+])
+def test_pos(mapping):
+    amounts = frozenset(amounts_from_map(mapping))
+    actual = +core.Balance(amounts)
+    assert set(actual.values()) == amounts
+
 @pytest.mark.parametrize('map1,map2,expected', [
     ({}, {}, True),
     ({}, {'USD': 0}, True),