addition and subtraction

fixedcal/core/date.py

@@ -116,5 +116,37 @@ class FixedDate:
             return self._day_of_year > o.day_of_year
         return self._year > o.year
 
-    def __sub__(self, o: "FixedDate") -> timedelta:
+    def __add__(self, o: timedelta) -> "FixedDate":
+        """Addition of FixedDate and timedelta.
+        Does not modify this instance, but creates new one.
+
+        Args:
+            o (timedelta): The time delta that will be added.
+
+        Returns:
+            FixedDate: New FixedDate instance that will hold the new date.
+        """
+        new_date = self.datetime + o
+        return FixedDate(date=new_date)
+
+    def __sub__(self, o):
+        """Subtraction of FixedDate and some other value.
+        Does not modify either one of the values.
+
+        Args:
+            o (Union[FixedDate, timedelta]): The value that will be added.
+
+        Raises:
+            ValueError: Given argument was not FixedDate nor timedelta.
+
+        Returns:
+            Union[FixedDate, timedelta]: With FixedDate as argument,
+            timedelta will be returned representing the difference of given fixed dates.
+            With timedelta as argument, new FixedDate will be returned.
+        """
+        if isinstance(o, FixedDate):
             return self.datetime - o.datetime
+        elif isinstance(o, timedelta):
+            new_date = self.datetime - o
+            return FixedDate(date=new_date)
+        raise ValueError("Invalid subtractor type, expected FixedDate or timedelta")

tests/operations_test.py

@@ -25,11 +25,34 @@ class TestOperations(unittest.TestCase):
     def test_less_than_with_true_expected(self):
         self.assertTrue(self.fixed1 < self.fixed2)
 
-    def test_subtration_with_two_different(self):
+    def test_subtration_of_two_dates(self):
         self.assertEqual(self.fixed2-self.fixed1, timedelta(1))
 
-    def test_subtration_with_smaller_first(self):
+    def test_subtration_of_two_dates_with_smaller_first(self):
         self.assertEqual(self.fixed1-self.fixed2, timedelta(-1))
 
-    def test_subtration_with_two_same(self):
+    def test_subtration_of_two_same_dates(self):
         self.assertEqual(self.fixed1-self.fixed1, timedelta(0))
+
+    def test_subtraction_of_timedelta(self):
+        result = self.fixed1 - timedelta(7)
+        self.assertEqual(result, FixedDate(date=datetime(2022, 11, 28)))
+
+    def test_subtraction_of_negative_timedelta(self):
+        result = self.fixed1 - timedelta(-2)
+        self.assertEqual(result, FixedDate(date=datetime(2022, 12, 7)))
+
+    def test_subtraction_of_invalid_type(self):
+        self.assertRaises(ValueError, lambda : self.fixed1 - 3)
+
+    def test_addition_of_timedelta(self):
+        result = self.fixed1 + timedelta(3)
+        self.assertEqual(result, FixedDate(date=datetime(2022, 12, 8)))
+
+    def test_addition_of_negative_timedelta(self):
+        result = self.fixed1 + timedelta(-3)
+        self.assertEqual(result, FixedDate(date=datetime(2022, 12, 2)))
+
+    def test_addition_does_not_modify(self):
+        _ = self.fixed1 + timedelta(2)
+        self.assertEqual(self.fixed1.datetime, datetime(2022, 12, 5))