Fixed WOM offset when n=0

doc/source/api.rst

@@ -2106,6 +2106,7 @@ Standard moving window functions
    Rolling.skew
    Rolling.kurt
    Rolling.apply
+   Rolling.aggregate
    Rolling.quantile
    Window.mean
    Window.sum
@@ -2133,6 +2134,7 @@ Standard expanding window functions
    Expanding.skew
    Expanding.kurt
    Expanding.apply
+   Expanding.aggregate
    Expanding.quantile
 
 Exponentially-weighted moving window functions

doc/source/whatsnew/v0.23.0.txt

@@ -438,6 +438,7 @@ Other Enhancements
   ``SQLAlchemy`` dialects supporting multivalue inserts include: ``mysql``, ``postgresql``, ``sqlite`` and any dialect with ``supports_multivalues_insert``. (:issue:`14315`, :issue:`8953`)
 - :func:`read_html` now accepts a ``displayed_only`` keyword argument to controls whether or not hidden elements are parsed (``True`` by default) (:issue:`20027`)
 - zip compression is supported via ``compression=zip`` in :func:`DataFrame.to_pickle`, :func:`Series.to_pickle`, :func:`DataFrame.to_csv`, :func:`Series.to_csv`, :func:`DataFrame.to_json`, :func:`Series.to_json`. (:issue:`17778`)
+- :class:`WeekOfMonth` constructor now supports ``n=0`` (:issue:`20517`).
 - :class:`DataFrame` and :class:`Series` now support matrix multiplication (```@```) operator (:issue:`10259`) for Python>=3.5
 - Updated ``to_gbq`` and ``read_gbq`` signature and documentation to reflect changes from
   the Pandas-GBQ library version 0.4.0. Adds intersphinx mapping to Pandas-GBQ
@@ -847,7 +848,7 @@ Other API Changes
 - :func:`DatetimeIndex.strftime` and :func:`PeriodIndex.strftime` now return an ``Index`` instead of a numpy array to be consistent with similar accessors (:issue:`20127`)
 - Constructing a Series from a list of length 1 no longer broadcasts this list when a longer index is specified (:issue:`19714`, :issue:`20391`).
 - :func:`DataFrame.to_dict` with ``orient='index'`` no longer casts int columns to float for a DataFrame with only int and float columns (:issue:`18580`)
-- A user-defined-function that is passed to :func:`Series.rolling().aggregate() <pandas.core.window.Rolling.aggregate>`, :func:`DataFrame.rolling().aggregate() <pandas.core.window.Rolling.aggregate>`, or its expanding cousins, will now *always* be passed a ``Series``, rather than an ``np.array``; ``.apply()`` only has the ``raw`` keyword, see :ref:`here <whatsnew_0230.enhancements.window_raw>`. This is consistent with the signatures of ``.aggregate()`` across pandas (:issue:`20584`)
+- A user-defined-function that is passed to :func:`Series.rolling().aggregate() <pandas.core.window.Rolling.aggregate>`, :func:`DataFrame.rolling().aggregate() <pandas.core.window.Rolling.aggregate>`, or its expanding cousins, will now *always* be passed a ``Series``, rather than a ``np.array``; ``.apply()`` only has the ``raw`` keyword, see :ref:`here <whatsnew_0230.enhancements.window_raw>`. This is consistent with the signatures of ``.aggregate()`` across pandas (:issue:`20584`)
 
 .. _whatsnew_0230.deprecations:
 

pandas/tests/indexes/datetimes/test_date_range.py

@@ -236,6 +236,12 @@ def test_catch_infinite_loop(self):
         pytest.raises(Exception, date_range, datetime(2011, 11, 11),
                       datetime(2011, 11, 12), freq=offset)
 
+    @pytest.mark.parametrize('periods', (1, 2))
+    def test_wom_len(self, periods):
+        # https://github.com/pandas-dev/pandas/issues/20517
+        res = date_range(start='20110101', periods=periods, freq='WOM-1MON')
+        assert len(res) == periods
+
 
 class TestGenRangeGeneration(object):
 

pandas/tests/tseries/offsets/test_offsets.py

@@ -2228,8 +2228,6 @@ class TestWeekOfMonth(Base):
     _offset = WeekOfMonth
 
     def test_constructor(self):
-        tm.assert_raises_regex(ValueError, "^N cannot be 0",
-                               WeekOfMonth, n=0, week=1, weekday=1)
         tm.assert_raises_regex(ValueError, "^Week", WeekOfMonth,
                                n=1, week=4, weekday=0)
         tm.assert_raises_regex(ValueError, "^Week", WeekOfMonth,
@@ -2261,6 +2259,19 @@ def test_offset(self):
             (-1, 2, 1, date3, datetime(2010, 12, 21)),
             (-1, 2, 1, date4, datetime(2011, 1, 18)),
 
+            (0, 0, 1, date1, datetime(2011, 1, 4)),
+            (0, 0, 1, date2, datetime(2011, 2, 1)),
+            (0, 0, 1, date3, datetime(2011, 2, 1)),
+            (0, 0, 1, date4, datetime(2011, 2, 1)),
+            (0, 1, 1, date1, datetime(2011, 1, 11)),
+            (0, 1, 1, date2, datetime(2011, 1, 11)),
+            (0, 1, 1, date3, datetime(2011, 2, 8)),
+            (0, 1, 1, date4, datetime(2011, 2, 8)),
+            (0, 0, 1, date1, datetime(2011, 1, 4)),
+            (0, 1, 1, date2, datetime(2011, 1, 11)),
+            (0, 2, 1, date3, datetime(2011, 1, 18)),
+            (0, 3, 1, date4, datetime(2011, 1, 25)),
+
             (1, 0, 0, date1, datetime(2011, 2, 7)),
             (1, 0, 0, date2, datetime(2011, 2, 7)),
             (1, 0, 0, date3, datetime(2011, 2, 7)),

pandas/tseries/offsets.py

@@ -1461,9 +1461,6 @@ def __init__(self, n=1, normalize=False, week=0, weekday=0):
         self.weekday = weekday
         self.week = week
 
-        if self.n == 0:
-            raise ValueError('N cannot be 0')
-
         if self.weekday < 0 or self.weekday > 6:
             raise ValueError('Day must be 0<=day<=6, got {day}'
                              .format(day=self.weekday))