reports: Add RelatedPostings.balance_at_cost() method.

This makes it easy to get results similar to `ledger -V`.

conservancy_beancount/reports/core.py

@@ -199,6 +199,16 @@ class RelatedPostings(Sequence[data.Posting]):
         except NameError:
             return Balance()
 
+    def balance_at_cost(self) -> Balance:
+        balance = MutableBalance()
+        for post in self:
+            if post.cost is None:
+                balance.add_amount(post.units)
+            else:
+                number = post.units.number * post.cost.number
+                balance.add_amount(data.Amount(number, post.cost.currency))
+        return balance
+
     def meta_values(self,
                     key: MetaKey,
                     default: Optional[MetaValue]=None,

tests/test_reports_related_postings.py

@@ -135,6 +135,61 @@ def test_iter_with_balance_credit_card(credit_card_cycle):
 def test_iter_with_balance_two_acccruals(two_accruals_three_payments):
     check_iter_with_balance(two_accruals_three_payments)
 
+def test_balance_at_cost_mixed():
+    txn = testutil.Transaction(postings=[
+        ('Expenses:Other', '22'),
+        ('Expenses:Other', '30', 'EUR', ('1.1',)),
+        ('Expenses:Other', '40', 'EUR'),
+        ('Expenses:Other', '50', 'USD', ('1.1', 'EUR')),
+    ])
+    related = core.RelatedPostings(data.Posting.from_txn(txn))
+    balance = related.balance_at_cost()
+    amounts = set(balance.values())
+    assert amounts == {testutil.Amount(55, 'USD'), testutil.Amount(95, 'EUR')}
+
+def test_balance_at_single_currency_cost():
+    txn = testutil.Transaction(postings=[
+        ('Expenses:Other', '22'),
+        ('Expenses:Other', '30', 'EUR', ('1.1',)),
+        ('Expenses:Other', '40', 'GBP', ('1.1',)),
+    ])
+    related = core.RelatedPostings(data.Posting.from_txn(txn))
+    balance = related.balance_at_cost()
+    amounts = set(balance.values())
+    assert amounts == {testutil.Amount(99)}
+
+def test_balance_at_cost_zeroed_out():
+    txn = testutil.Transaction(postings=[
+        ('Income:Other', '-22'),
+        ('Assets:Receivable:Accounts', '20', 'EUR', ('1.1',)),
+    ])
+    related = core.RelatedPostings(data.Posting.from_txn(txn))
+    balance = related.balance_at_cost()
+    assert balance.is_zero()
+
+def test_balance_at_cost_singleton():
+    txn = testutil.Transaction(postings=[
+        ('Assets:Receivable:Accounts', '20', 'EUR', ('1.1',)),
+    ])
+    related = core.RelatedPostings(data.Posting.from_txn(txn))
+    balance = related.balance_at_cost()
+    amounts = set(balance.values())
+    assert amounts == {testutil.Amount(22)}
+
+def test_balance_at_cost_singleton_without_cost():
+    txn = testutil.Transaction(postings=[
+        ('Assets:Receivable:Accounts', '20'),
+    ])
+    related = core.RelatedPostings(data.Posting.from_txn(txn))
+    balance = related.balance_at_cost()
+    amounts = set(balance.values())
+    assert amounts == {testutil.Amount(20)}
+
+def test_balance_at_cost_empty():
+    related = core.RelatedPostings()
+    balance = related.balance_at_cost()
+    assert balance.is_zero()
+
 def test_meta_values_empty():
     related = core.RelatedPostings()
     assert related.meta_values('key') == set()