cliutil: Add date_arg() function.

Meant to be used as an argument type.

conservancy_beancount/cliutil.py

@@ -17,6 +17,7 @@ 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 argparse
+import datetime
 import enum
 import io
 import logging
@@ -231,6 +232,9 @@ def add_version_argument(parser: argparse.ArgumentParser) -> argparse.Action:
         help="Show program version and license information",
     )
 
+def date_arg(arg: str) -> datetime.date:
+    return datetime.datetime.strptime(arg, '%Y-%m-%d').date()
+
 def make_entry_point(mod_name: str, prog_name: str=sys.argv[0]) -> Callable[[], int]:
     """Create an entry_point function for a tool
 

tests/test_cliutil.py

@@ -15,6 +15,7 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 import argparse
+import datetime
 import errno
 import io
 import inspect
@@ -74,6 +75,26 @@ def test_bytes_output_stream(path):
     actual = cliutil.bytes_output(path, stream)
     assert actual is stream
 
+@pytest.mark.parametrize('year,month,day', [
+    (2000, 1, 1),
+    (2016, 2, 29),
+    (2020, 12, 31),
+])
+def test_date_arg_valid(year, month, day):
+    arg = f'{year}-{month}-{day}'
+    expected = datetime.date(year, month, day)
+    assert cliutil.date_arg(arg) == expected
+
+@pytest.mark.parametrize('arg', [
+    '2000',
+    '20-02-12',
+    '2019-02-29',
+    'two thousand',
+])
+def test_date_arg_invalid(arg):
+    with pytest.raises(ValueError):
+        cliutil.date_arg(arg)
+
 @pytest.mark.parametrize('func_name', [
     'bytes_output',
     'text_output',