add day_specifier to recurrence

Add a new property to the recurrence that can be used to generate
grammatically correct versions of phrases like 'Monthly on the first
Thursday'.

Change-Id: I2ebfd60ebfbbc499ed08f6f63e91cbb87c8691b1
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

yaml2ical/recurrence.py

@@ -39,6 +39,19 @@ class _Recurrence(object, metaclass=abc.ABCMeta):
     def __str__(self):
         "Return string representation of the recurrence rule"
 
+    @property
+    def day_specifier(self):
+        """Return string prefix for day.
+
+        For example, monthly recurring events may return 'first' to
+        indicate the first instance of a particular day of the month
+        (e.g., first Thursday).
+        """
+        # NOTE(dhellmann): This is not an abstract property because
+        # most of the subclasses will use this concrete
+        # implementation.
+        return ''
+
 
 class WeeklyRecurrence(_Recurrence):
     """Meetings occuring every week."""
@@ -213,6 +226,18 @@ class MonthlyRecurrence(_Recurrence):
     def __str__(self):
         return "Monthly"
 
+    _ORDINALS = [
+        'first',
+        'second',
+        'third',
+        'fourth',
+        'fifth',
+    ]
+
+    @property
+    def day_specifier(self):
+        return 'the {}'.format(self._ORDINALS[self._week - 1])
+
 
 supported_recurrences = {
     'weekly': WeeklyRecurrence(),

yaml2ical/tests/test_recurrence.py

@@ -105,3 +105,15 @@ class RecurrenceTestCase(unittest.TestCase):
             self.next_meeting,
             rec,
         )
+
+    def test_monthly_day_specifier(self):
+        weeks = [
+            (1, 'the first'),
+            (2, 'the second'),
+            (3, 'the third'),
+            (4, 'the fourth'),
+            (5, 'the fifth'),
+        ]
+        for i, expected in weeks:
+            rec = recurrence.MonthlyRecurrence(week=i, day='Thursday')
+            self.assertEqual(expected, rec.day_specifier)