balance_sheet: Balance only considers post_type for Expenses.

This simplifies the code and slightly optimizes it, since now Balance
won't store and keep re-summing income-type breakdowns that nothing
needs.

conservancy_beancount/reports/balance_sheet.py

@@ -78,11 +78,6 @@ class BalanceKey(NamedTuple):
 
 
 class Balances:
-    POST_TYPES = {
-        'Income': 'income-type',
-        'Expenses': 'expense-type',
-    }
-
     def __init__(self,
                  postings: Iterable[data.Posting],
                  start_date: datetime.date,
@@ -124,9 +119,9 @@ class Balances:
                     raise TypeError()
             except (KeyError, TypeError):
                 classification = account
-            try:
-                post_type = post.meta[self.POST_TYPES[account.root_part()]]
-            except KeyError:
+            if account.root_part() == 'Expenses':
+                post_type = post.meta.get('expense-type')
+            else:
                 post_type = None
             key = BalanceKey(account, classification, period, fund, post_type)
             self.balances[key] += post.at_cost()

tests/test_reports_balance_sheet.py

@@ -87,13 +87,11 @@ def income_expense_balances():
     ({'period': Period.PERIOD, 'account': 'Expenses'}, 26),
     ({'fund': Fund.RESTRICTED, 'account': 'Income'}, -10),
     ({'fund': Fund.UNRESTRICTED, 'account': 'Expenses'}, 25),
-    ({'post_type': 'Donations'}, -10),
     ({'post_type': 'fundraising'}, 20),
     ({'post_type': 'management'}, 10),
     ({'post_type': 'Nonexistent'}, None),
-    ({'period': Period.PRIOR, 'post_type': 'RBI'}, '-4.80'),
+    ({'period': Period.PRIOR, 'post_type': 'fundraising'}, '9.60'),
     ({'fund': Fund.RESTRICTED, 'post_type': 'program'}, 10),
-    ({'period': Period.PERIOD, 'fund': Fund.UNRESTRICTED, 'post_type': 'RBI'}, '-2.60'),
     ({'period': Period.PRIOR, 'fund': Fund.RESTRICTED, 'post_type': 'program'}, '4.80'),
     ({'period': Period.PERIOD, 'fund': Fund.RESTRICTED, 'post_type': 'ø'}, None),
 ])