Repository: peopledoc/workalendar (python)

Issue:
Shift mechanism for US calendars are not tested
They are tested by the state calendars when some rules apply (shift exceptions here and there, etc), but these rules were not tested independantly.

Pull request description:
Added isolated tests for shifting rules for US Calendars
These shift mechanisms were previously tested via the regular US calendars, but I felt like we needed to have dedicated isolated tests.

- [x] Tests with a significant number of years to be tested for your calendar.
- [x] Changelog amended with a mention describing your changes.

closes #335 

Patch:
diff --git a/Changelog.md b/Changelog.md
index ee439b0..cac7489 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -2,7 +2,7 @@
 
 ## master (unreleased)
 
-Nothing here yet.
+- Added isolated tests for shifting mechanics in USA calendars - previously untested (#335).
 
 ## v4.2.0 (2019-02-21)
 
diff --git a/workalendar/usa/core.py b/workalendar/usa/core.py
index 4dd52fd..4caaa7b 100644
--- a/workalendar/usa/core.py
+++ b/workalendar/usa/core.py
@@ -103,10 +103,18 @@ class UnitedStates(WesternCalendar, ChristianMixin):
         if christmas_eve in holiday_lookup:
             # You are observing the THU before, as an extra XMas Eve
             if christmas.weekday() == SAT:
+                # Remove the "fake" XMAS Day shift, the one done before.
+                new_holidays.remove(
+                    (christmas_eve, "Christmas Day (Observed)")
+                )
                 new_holidays.append((date(year, 12, 23),
                                      "Christmas Eve (Observed)"))
-            # You are observing the 26th (TUE) and 27th (WED)
+            # You are observing the 26th (TUE)
             elif christmas.weekday() == MON:
+                # Remove the "fake" XMAS Eve shift, done before
+                new_holidays.remove(
+                    (christmas, "Christmas Eve (Observed)")
+                )
                 new_holidays.append((date(year, 12, 26),
                                      "Christmas Day (Observed)"))
         return holidays + new_holidays


Test patch:
diff --git a/workalendar/tests/test_usa.py b/workalendar/tests/test_usa.py
index 97af160..d7fbc1a 100644
--- a/workalendar/tests/test_usa.py
+++ b/workalendar/tests/test_usa.py
@@ -1423,3 +1423,117 @@ class Guam(UnitedStatesTest):
             holidays_dict[date(2019, 12, 8)],
             "Lady of Camarin Day"
         )
+
+
+class NormalShiftTestCase(UnitedStatesTest):
+    # Using a fake calendar here
+    class NormalShiftUnitedStates(UnitedStates):
+        "Normal Shift Fake United State calendar"
+        include_christmas_eve = True
+
+    cal_class = NormalShiftUnitedStates
+
+    def test_shift_2015(self):
+        # Test a normal shift on 4th of July.
+        # 2015: Happens on a Saturday, observed on FRI
+        holidays = self.cal.holidays(2015)
+        holiday_dict = dict(holidays)
+        fourth_july = date(2015, 7, 4)
+        observed = date(2015, 7, 3)
+        self.assertIn(fourth_july, holiday_dict)
+        self.assertEqual(holiday_dict[fourth_july], "Independence Day")
+        self.assertIn(observed, holiday_dict)
+        self.assertEqual(holiday_dict[observed], "Independence Day (Observed)")
+
+    def test_shift_2010(self):
+        # Test a normal shift on 4th of July.
+        # 2010: Happens on a SUN, observed on MON
+        holidays = self.cal.holidays(2010)
+        holiday_dict = dict(holidays)
+        fourth_july = date(2010, 7, 4)
+        observed = date(2010, 7, 5)
+        self.assertIn(fourth_july, holiday_dict)
+        self.assertEqual(holiday_dict[fourth_july], "Independence Day")
+        self.assertIn(observed, holiday_dict)
+        self.assertEqual(holiday_dict[observed], "Independence Day (Observed)")
+
+    def test_new_years_shift(self):
+        # If January, 1st *of the year after* happens on SAT, add New Years Eve
+        holidays = self.cal.holidays(2010)
+        holiday_dict = dict(holidays)
+        new_years_eve = date(2010, 12, 31)
+        self.assertIn(new_years_eve, holiday_dict)
+        self.assertEqual(
+            holiday_dict[new_years_eve],
+            "New Years Day (Observed)"
+        )
+        # The year after, it's not shifted
+        holidays = self.cal.holidays_set(2011)
+        new_years_eve = date(2011, 12, 31)
+        self.assertNotIn(new_years_eve, holidays)
+
+    def test_christmas_extra_shift_2010(self):
+        # XMAs Eve is included. *and* XMas falls on SAT.
+        # So you have the following holidays:
+        # * 24th & 25th (XMas Eve and XMas day)
+        # * 27th (XMas Shift)
+        # * 23rd (XMas Eve shifted on THU)
+        holidays = self.cal.holidays(2010)
+        holiday_dict = dict(holidays)
+        dec_23rd = date(2010, 12, 23)
+        dec_24th = date(2010, 12, 24)
+        dec_25th = date(2010, 12, 25)
+        for day in (dec_23rd, dec_24th, dec_25th):
+            self.assertIn(day, holiday_dict)
+        self.assertEqual(holiday_dict[dec_23rd], "Christmas Eve (Observed)")
+        self.assertEqual(holiday_dict[dec_24th], "Christmas Eve")
+        self.assertEqual(holiday_dict[dec_25th], "Christmas Day")
+
+    def test_christmas_extra_shift_2006(self):
+        # XMAs Eve is included. *and* XMas falls on MON.
+        # So you have the following holidays:
+        # * 24th & 25th (XMas Eve and XMas day)
+        # * 26th (XMas Shift)
+        holidays = self.cal.holidays(2006)
+        holiday_dict = dict(holidays)
+        dec_24th = date(2006, 12, 24)
+        dec_25th = date(2006, 12, 25)
+        dec_26th = date(2006, 12, 26)
+        for day in (dec_24th, dec_25th, dec_26th):
+            self.assertIn(day, holiday_dict)
+        self.assertEqual(holiday_dict[dec_24th], "Christmas Eve")
+        self.assertEqual(holiday_dict[dec_25th], "Christmas Day")
+        self.assertEqual(holiday_dict[dec_26th], "Christmas Day (Observed)")
+
+
+class NormalShiftTestCaseExceptions(UnitedStatesTest):
+    # Using a fake calendar here
+    class NormalShiftUnitedStatesExceptions(UnitedStates):
+        "Normal Shift Fake United State calendar"
+        shift_exceptions = (
+            (7, 4),  # Month/Day == Fourth of July.
+        )
+
+    cal_class = NormalShiftUnitedStatesExceptions
+
+    def test_shift_2015(self):
+        # Test a normal shift on 4th of July.
+        # 2015: Happens on a Saturday, not shifted
+        holidays = self.cal.holidays(2015)
+        holiday_dict = dict(holidays)
+        fourth_july = date(2015, 7, 4)
+        observed = date(2015, 7, 3)
+        self.assertIn(fourth_july, holiday_dict)
+        self.assertEqual(holiday_dict[fourth_july], "Independence Day")
+        self.assertNotIn(observed, holiday_dict)
+
+    def test_shift_2010(self):
+        # Test a normal shift on 4th of July.
+        # 2010: Happens on a SUN, not shifted
+        holidays = self.cal.holidays(2010)
+        holiday_dict = dict(holidays)
+        fourth_july = date(2010, 7, 4)
+        observed = date(2010, 7, 5)
+        self.assertIn(fourth_july, holiday_dict)
+        self.assertEqual(holiday_dict[fourth_july], "Independence Day")
+        self.assertNotIn(observed, holiday_dict)