books: Add date-fetching methods to FiscalYear.

conservancy_beancount/books.py

@@ -50,6 +50,19 @@ class FiscalYear(NamedTuple):
         else:
             return date.year
 
+    def first_date(self, year: Year) -> datetime.date:
+        if isinstance(year, datetime.date):
+            year = self.for_date(year)
+        return datetime.date(year, self.month, self.day)
+
+    def last_date(self, year: Year) -> datetime.date:
+        return self.next_fy_date(year) - datetime.timedelta(days=1)
+
+    def next_fy_date(self, year: Year) -> datetime.date:
+        if isinstance(year, datetime.date):
+            year = self.for_date(year)
+        return datetime.date(year + 1, self.month, self.day)
+
     def range(self, from_fy: Year, to_fy: Optional[Year]=None) -> Iterable[int]:
         """Return a range of fiscal years
 

tests/test_books_fiscal_year.py

@@ -134,3 +134,53 @@ def test_range_offset_only(cy_fy, year_offset):
         expected = range(year, year + year_offset + 1)
     actual = list(cy_fy.range(year_offset))
     assert actual == list(expected)
+
+@pytest.mark.parametrize('year', range(2016, 2022))
+def test_first_date_year_arg(conservancy_fy, year):
+    assert conservancy_fy.first_date(year) == datetime.date(year, 3, 1)
+
+@pytest.mark.parametrize('date', [
+    datetime.date(2019, 1, 1),
+    datetime.date(2019, 10, 10),
+    datetime.date(2020, 2, 2),
+    datetime.date(2020, 12, 12),
+])
+def test_first_date_date_arg(conservancy_fy, date):
+    year = date.year
+    if date.month < 3:
+        year -= 1
+    assert conservancy_fy.first_date(date) == datetime.date(year, 3, 1)
+
+@pytest.mark.parametrize('year', range(2016, 2022))
+def test_last_date_year_arg(conservancy_fy, year):
+    day = 28 if year % 4 else 29
+    assert conservancy_fy.last_date(year - 1) == datetime.date(year, 2, day)
+
+@pytest.mark.parametrize('date', [
+    datetime.date(2019, 1, 1),
+    datetime.date(2019, 10, 10),
+    datetime.date(2020, 2, 2),
+    datetime.date(2020, 12, 12),
+])
+def test_last_date_date_arg(conservancy_fy, date):
+    year = date.year
+    if date.month >= 3:
+        year += 1
+    day = 28 if year % 4 else 29
+    assert conservancy_fy.last_date(date) == datetime.date(year, 2, day)
+
+@pytest.mark.parametrize('year', range(2016, 2022))
+def test_next_fy_date_year_arg(conservancy_fy, year):
+    assert conservancy_fy.next_fy_date(year) == datetime.date(year + 1, 3, 1)
+
+@pytest.mark.parametrize('date', [
+    datetime.date(2019, 1, 1),
+    datetime.date(2019, 10, 10),
+    datetime.date(2020, 2, 29),
+    datetime.date(2020, 12, 12),
+])
+def test_next_fy_date_date_arg(conservancy_fy, date):
+    year = date.year
+    if date.month >= 3:
+        year += 1
+    assert conservancy_fy.next_fy_date(date) == datetime.date(year, 3, 1)