Make DateOffset.kwds a property (pandas-dev#19403)

Author: jbrockmendel

doc/source/whatsnew/v0.23.0.txt

@@ -341,6 +341,7 @@ Other API Changes
 - :func:`DatetimeIndex.shift` and :func:`TimedeltaIndex.shift` will now raise ``NullFrequencyError`` (which subclasses ``ValueError``, which was raised in older versions) when the index object frequency is ``None`` (:issue:`19147`)
 - Addition and subtraction of ``NaN`` from a :class:`Series` with ``dtype='timedelta64[ns]'`` will raise a ``TypeError` instead of treating the ``NaN`` as ``NaT`` (:issue:`19274`)
 - Set operations (union, difference...) on :class:`IntervalIndex` with incompatible index types will now raise a ``TypeError`` rather than a ``ValueError`` (:issue:`19329`)
+- :class:`DateOffset` objects render more simply, e.g. "<DateOffset: days=1>" instead of "<DateOffset: kwds={'days': 1}>" (:issue:`19403`)
 
 .. _whatsnew_0230.deprecations:
 

pandas/_libs/tslibs/offsets.pyx

@@ -302,6 +302,14 @@ class _BaseOffset(object):
     _normalize_cache = True
     _cacheable = False
     _day_opt = None
+    _attributes = frozenset(['n', 'normalize'])
+
+    @property
+    def kwds(self):
+        # for backwards-compatibility
+        kwds = {name: getattr(self, name, None) for name in self._attributes
+                if name not in ['n', 'normalize']}
+        return {name: kwds[name] for name in kwds if kwds[name] is not None}
 
     def __call__(self, other):
         return self.apply(other)

pandas/core/indexes/datetimes.py

@@ -71,9 +71,11 @@ def f(self):
             if field in ['is_month_start', 'is_month_end',
                          'is_quarter_start', 'is_quarter_end',
                          'is_year_start', 'is_year_end']:
-                month_kw = (self.freq.kwds.get('startingMonth',
-                                               self.freq.kwds.get('month', 12))
-                            if self.freq else 12)
+                freq = self.freq
+                month_kw = 12
+                if freq:
+                    kwds = freq.kwds
+                    month_kw = kwds.get('startingMonth', kwds.get('month', 12))
 
                 result = fields.get_start_end_field(values, field,
                                                     self.freqstr, month_kw)

pandas/tests/tseries/offsets/test_offsets.py

@@ -218,7 +218,7 @@ def test_offset_freqstr(self, offset_types):
 
         freqstr = offset.freqstr
         if freqstr not in ('<Easter>',
-                           "<DateOffset: kwds={'days': 1}>",
+                           "<DateOffset: days=1>",
                            'LWOM-SAT', ):
             code = get_offset(freqstr)
             assert offset.rule_code == code