bugfix for FY5253 case with bunched yearends (#18550)

Author: jbrockmendel

doc/source/whatsnew/v0.22.0.txt

@@ -265,3 +265,5 @@ Other
 - Fixed a bug where creating a Series from an array that contains both tz-naive and tz-aware values will result in a Series whose dtype is tz-aware instead of object (:issue:`16406`)
 - Fixed construction of a :class:`Series` from a ``dict`` containing ``NaN`` as key (:issue:`18480`)
 - Adding a ``Period`` object to a ``datetime`` or ``Timestamp`` object will now correctly raise a ``TypeError`` (:issue:`17983`)
+- Fixed a bug where ``FY5253`` date offsets could incorrectly raise an ``AssertionError`` in arithmetic operatons (:issue:`14774`)
+-

pandas/tests/tseries/offsets/test_fiscal.py

@@ -9,6 +9,7 @@
 
 import pandas.util.testing as tm
 
+from pandas import Timestamp
 from pandas.tseries.frequencies import get_offset, _INVALID_FREQ_ERROR
 from pandas.tseries.offsets import FY5253Quarter, FY5253
 from pandas._libs.tslibs.offsets import WeekDay
@@ -604,3 +605,23 @@ def test_offset(self):
         assert_offset_equal(offset2,
                             datetime(2013, 1, 15),
                             datetime(2013, 3, 30))
+
+
+def test_bunched_yearends():
+    # GH#14774 cases with two fiscal year-ends in the same calendar-year
+    fy = FY5253(n=1, weekday=5, startingMonth=12, variation='nearest')
+    dt = Timestamp('2004-01-01')
+    assert fy.rollback(dt) == Timestamp('2002-12-28')
+    assert (-fy).apply(dt) == Timestamp('2002-12-28')
+    assert dt - fy == Timestamp('2002-12-28')
+
+    assert fy.rollforward(dt) == Timestamp('2004-01-03')
+    assert fy.apply(dt) == Timestamp('2004-01-03')
+    assert fy + dt == Timestamp('2004-01-03')
+    assert dt + fy == Timestamp('2004-01-03')
+
+    # Same thing, but starting from a Timestamp in the previous year.
+    dt = Timestamp('2003-12-31')
+    assert fy.rollback(dt) == Timestamp('2002-12-28')
+    assert (-fy).apply(dt) == Timestamp('2002-12-28')
+    assert dt - fy == Timestamp('2002-12-28')

pandas/tseries/offsets.py

@@ -1831,21 +1831,26 @@ def apply(self, other):
         elif n > 0:
             if other < prev_year:
                 n -= 2
-                # TODO: Not hit in tests
-            elif other < cur_year:
+            elif prev_year < other < cur_year:
                 n -= 1
-            elif other < next_year:
+            elif cur_year < other < next_year:
                 pass
             else:
                 assert False
         else:
-            if other > next_year:
+            if next_year < other:
                 n += 2
-                # TODO: Not hit in tests
-            elif other > cur_year:
+                # TODO: Not hit in tests; UPDATE: looks impossible
+            elif cur_year < other < next_year:
                 n += 1
-            elif other > prev_year:
+            elif prev_year < other < cur_year:
                 pass
+            elif (other.year == prev_year.year and other < prev_year and
+                  prev_year - other <= timedelta(6)):
+                # GH#14774, error when next_year.year == cur_year.year
+                # e.g. prev_year == datetime(2004, 1, 3),
+                # other == datetime(2004, 1, 1)
+                n -= 1
             else:
                 assert False