reports: Add PeriodPostings class.

This is AccountPostings from the ledger report, cleaned up to be more
general.

conservancy_beancount/reports/core.py

@@ -393,6 +393,57 @@ class RelatedPostings(Sequence[data.Posting]):
         return {post.meta.get(key, default) for post in self}
 
 
+class PeriodPostings(RelatedPostings):
+    """Postings filtered and balanced over a date range
+
+    Create a subclass with ``PeriodPostings.with_start_date(date)``.
+    Note that there is no explicit stop date. The expectation is that the
+    caller has already filtered out posts past the stop date from the input.
+
+    Instances of that subclass will have three Balance attributes:
+
+    * ``start_bal`` is the balance at cost of postings to your start date
+    * ``period_bal`` is the balance at cost of postings from your start date
+    * ``stop_bal`` is the balance at cost of all postings
+
+    Use this subclass when your report includes a lot of balances over time to
+    help you get the math right.
+    """
+    __slots__ = (
+        'begin_bal',
+        'end_bal',
+        'period_bal',
+        'start_bal',
+        'stop_bal',
+    )
+    START_DATE = datetime.date(datetime.MINYEAR, 1, 1)
+
+    def __init__(self,
+                 source: Iterable[data.Posting]=(),
+                 *,
+                 _can_own: bool=False,
+    ) -> None:
+        start_posts: List[data.Posting] = []
+        period_posts: List[data.Posting] = []
+        for post in source:
+            if post.meta.date < self.START_DATE:
+                start_posts.append(post)
+            else:
+                period_posts.append(post)
+        super().__init__(period_posts, _can_own=True)
+        self.start_bal = RelatedPostings(start_posts, _can_own=True).balance_at_cost()
+        self.period_bal = self.balance_at_cost()
+        self.stop_bal = self.start_bal + self.period_bal
+        # Convenience aliases
+        self.begin_bal = self.start_bal
+        self.end_bal = self.stop_bal
+
+    @classmethod
+    def with_start_date(cls: Type[RelatedType], start_date: datetime.date) -> Type[RelatedType]:
+        name = f'BalancePostings{start_date.strftime("%Y%m%d")}'
+        return type(name, (cls,), {'START_DATE': start_date})
+
+
 class BaseSpreadsheet(Generic[RT, ST], metaclass=abc.ABCMeta):
     """Abstract base class to help write spreadsheets
 

conservancy_beancount/reports/ledger.py

@@ -85,20 +85,6 @@ PostTally = List[Tuple[int, data.Account]]
 PROGNAME = 'ledger-report'
 logger = logging.getLogger('conservancy_beancount.reports.ledger')
 
-class AccountPostings(core.RelatedPostings):
-    START_DATE: datetime.date
-
-    def __init__(self,
-                 source: Iterable[data.Posting]=(),
-                 *,
-                 _can_own: bool=False,
-    ) -> None:
-        super().__init__(source, _can_own=_can_own)
-        self.start_bal = self.balance_at_cost_by_date(self.START_DATE)
-        self.stop_bal = self.balance_at_cost()
-        self.period_bal = self.stop_bal - self.start_bal
-
-
 class LedgerODS(core.BaseODS[data.Posting, data.Account]):
     CORE_COLUMNS: Sequence[str] = [
         'Date',
@@ -310,9 +296,7 @@ class LedgerODS(core.BaseODS[data.Posting, data.Account]):
         self._report_section_balance(key, 'stop')
 
     def write_row(self, row: data.Posting) -> None:
-        if row.meta.date not in self.date_range:
-            return
-        elif row.cost is None:
+        if row.cost is None:
             amount_cell = odf.table.TableCell()
         else:
             amount_cell = self.currency_cell(self.norm_func(row.units))
@@ -378,13 +362,13 @@ class LedgerODS(core.BaseODS[data.Posting, data.Account]):
         self._combined_balance_row(balance_accounts, 'stop')
 
     def write(self, rows: Iterable[data.Posting]) -> None:
-        AccountPostings.START_DATE = self.date_range.start
-        self.account_groups = dict(AccountPostings.group_by_account(
+        related_cls = core.PeriodPostings.with_start_date(self.date_range.start)
+        self.account_groups = dict(related_cls.group_by_account(
             post for post in rows if post.meta.date < self.date_range.stop
         ))
         self.write_balance_sheet()
         tally_by_account_iter = (
-            (account, sum(1 for post in related if post.meta.date in self.date_range))
+            (account, len(related))
             for account, related in self.account_groups.items()
         )
         tally_by_account = {

tests/test_reports_period_postings.py

@@ -0,0 +1,71 @@
+"""test_reports_period_postings - Unit tests for PeriodPostings"""
+# Copyright © 2020  Brett Smith
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+import datetime
+import operator
+
+from decimal import Decimal
+
+import pytest
+
+from . import testutil
+
+from conservancy_beancount import data
+from conservancy_beancount.reports import core
+
+def check_balance_attrs(balance_postings, start_usd, stop_usd):
+    if start_usd:
+        expected = {'USD': testutil.Amount(start_usd)}
+        assert balance_postings.start_bal == expected
+        assert balance_postings.begin_bal == expected
+    else:
+        assert balance_postings.start_bal.is_zero()
+        assert balance_postings.begin_bal.is_zero()
+    expected = {'USD': testutil.Amount(stop_usd)}
+    assert balance_postings.stop_bal == expected
+    assert balance_postings.end_bal == expected
+    expected = {'USD': testutil.Amount(stop_usd - start_usd)}
+    assert balance_postings.period_bal == expected
+
+@pytest.mark.parametrize('start_date,expect_start_bal', [
+    (datetime.date(2019, 2, 1), 0),
+    (datetime.date(2019, 4, 1), 30),
+    (datetime.date(2019, 6, 1), 120),
+])
+def test_balance_postings_attrs(start_date, expect_start_bal):
+    entries = [testutil.Transaction(date=datetime.date(2019, n, 15), postings=[
+        ('Income:Donations', -n * 10),
+        ('Assets:Cash', n * 10),
+    ]) for n in range(3, 7)]
+    cls = core.PeriodPostings.with_start_date(start_date)
+    actual = dict(cls.group_by_account(data.Posting.from_entries(entries)))
+    assert len(actual) == 2
+    check_balance_attrs(actual['Assets:Cash'], expect_start_bal, 180)
+    check_balance_attrs(actual['Income:Donations'], -expect_start_bal, -180)
+
+@pytest.mark.parametrize('start_date,expect_count', [
+    (datetime.date(2019, 2, 1), 4),
+    (datetime.date(2019, 4, 1), 3),
+    (datetime.date(2019, 6, 1), 1),
+])
+def test_balance_postings_filter(start_date, expect_count):
+    entries = [testutil.Transaction(date=datetime.date(2019, n, 15), postings=[
+        ('Income:Donations', -n * 10),
+        ('Assets:Cash', n * 10),
+    ]) for n in range(3, 7)]
+    cls = core.PeriodPostings.with_start_date(start_date)
+    for _, related in cls.group_by_account(data.Posting.from_entries(entries)):
+        assert len(related) == expect_count