fix Series.hasnans to not cache pandas-dev#19700

doc/source/whatsnew/v0.24.0.txt

@@ -369,6 +369,7 @@ Missing
 ^^^^^^^
 
 - Bug in :func:`DataFrame.fillna` where a ``ValueError`` would raise when one column contained a ``datetime64[ns, tz]`` dtype (:issue:`15522`)
+- Bug in :func:`Series.hasnans` that could be incorrectly cached and return incorrect answers if null elements are introduced after an initial call (:issue:`19700`)
 
 MultiIndex
 ^^^^^^^^^^

pandas/core/series.py

@@ -166,6 +166,10 @@ class Series(base.IndexOpsMixin, generic.NDFrame):
         ['asobject', 'sortlevel', 'reshape', 'get_value', 'set_value',
          'from_csv', 'valid'])
 
+    # Override cache_readonly bc Series is mutable
+    hasnans = property(base.IndexOpsMixin.hasnans.func,
+                       doc=base.IndexOpsMixin.hasnans.__doc__)
+
     def __init__(self, data=None, index=None, dtype=None, name=None,
                  copy=False, fastpath=False):
 

pandas/tests/series/test_internals.py

@@ -11,6 +11,7 @@
 from pandas import Series
 from pandas.core.indexes.datetimes import Timestamp
 import pandas._libs.lib as lib
+import pandas as pd
 
 from pandas.util.testing import assert_series_equal
 import pandas.util.testing as tm
@@ -309,3 +310,16 @@ def test_convert_preserve_all_bool(self):
         r = s._convert(datetime=True, numeric=True)
         e = Series([False, True, False, False], dtype=bool)
         tm.assert_series_equal(r, e)
+
+
+def test_hasnans_unchached_for_series():
+    # GH#19700
+    idx = pd.Index([0, 1])
+    assert not idx.hasnans
+    assert 'hasnans' in idx._cache
+    ser = idx.to_series()
+    assert not ser.hasnans
+    assert not hasattr(ser, '_cache')
+    ser.iloc[-1] = np.nan
+    assert ser.hasnans
+    assert pd.Series.hasnans.__doc__ == pd.Index.hasnans.__doc__