data: Add Posting.at_cost() method.

conservancy_beancount/data.py

@@ -408,6 +408,12 @@ class Posting(BasePosting):
             except AttributeError:
                 pass
 
+    def at_cost(self) -> Amount:
+        if self.cost is None:
+            return self.units
+        else:
+            return Amount(self.units.number * self.cost.number, self.cost.currency)
+
 
 _KT = TypeVar('_KT', bound=Hashable)
 _VT = TypeVar('_VT')

conservancy_beancount/reports/core.py

@@ -356,22 +356,10 @@ class RelatedPostings(Sequence[data.Posting]):
             yield post, balance
 
     def balance(self) -> Balance:
-        for _, balance in self.iter_with_balance():
-            pass
-        try:
-            return balance
-        except NameError:
-            return Balance()
+        return Balance(post.units for post in self)
 
     def balance_at_cost(self) -> Balance:
-        balance = MutableBalance()
-        for post in self:
-            if post.cost is None:
-                balance += post.units
-            else:
-                number = post.units.number * post.cost.number
-                balance += data.Amount(number, post.cost.currency)
-        return balance
+        return Balance(post.at_cost() for post in self)
 
     def meta_values(self,
                     key: MetaKey,

tests/test_data_posting.py

@@ -82,3 +82,27 @@ def test_from_entries_mix_txns_and_other_directives(simple_txn):
         assert all(source[x] == post[x] for x in range(len(source) - 1))
         assert isinstance(post.account, data.Account)
         assert post.meta['note']  # Only works with PostingMeta
+
+@pytest.mark.parametrize('cost_num', [105, 110, 115])
+def test_at_cost(cost_num):
+    post = data.Posting(
+        'Income:Donations',
+        testutil.Amount(25, 'EUR'),
+        testutil.Cost(cost_num, 'JPY'),
+        None,
+        '*',
+        None,
+    )
+    assert post.at_cost() == testutil.Amount(25 * cost_num, 'JPY')
+
+def test_at_cost_no_cost():
+    amount = testutil.Amount(25, 'EUR')
+    post = data.Posting(
+        'Income:Donations',
+        amount,
+        None,
+        None,
+        '*',
+        None,
+    )
+    assert post.at_cost() == amount