Implement maybe_cache for compat between immutable/mutable classes

doc/source/whatsnew/v0.23.0.txt

@@ -848,3 +848,4 @@ Other
 ^^^^^
 
 - Improved error message when attempting to use a Python keyword as an identifier in a ``numexpr`` backed query (:issue:`18221`)
+- 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`)

pandas/_libs/properties.pyx

@@ -9,22 +9,30 @@ from cpython cimport (
 cdef class cache_readonly(object):
 
     cdef readonly:
-        object func, name, allow_setting
+        object func, name, allow_setting, __doc__
 
     def __init__(self, func=None, allow_setting=False):
         if func is not None:
             self.func = func
             self.name = func.__name__
+            self.__doc__ = func.__doc__
         self.allow_setting = allow_setting
 
     def __call__(self, func, doc=None):
         self.func = func
         self.name = func.__name__
+        if doc is not None:
+            self.__doc__ = doc
+        else:
+            self.__doc__ = func.__doc__
         return self
 
     def __get__(self, obj, typ):
-        # Get the cache or set a default one if needed
+        if obj is None:
+            # accessed on the class, not the instance
+            return self
 
+        # Get the cache or set a default one if needed
         cache = getattr(obj, '_cache', None)
         if cache is None:
             try:
@@ -55,6 +63,66 @@ cdef class cache_readonly(object):
 
         PyDict_SetItem(cache, self.name, value)
 
+
+cdef class maybe_cache(object):
+    """
+    A configurable property-like class that acts like a cache_readonly
+    if an "_immutable" flag is True and a like property otherwise
+    """
+
+    cdef readonly:
+        object func, name, __doc__
+
+    def __init__(self, func=None):
+        if func is not None:
+            self.func = func
+            self.name = func.__name__
+            self.__doc__ = func.__doc__
+
+    def __call__(self, func, doc=None):
+        self.func = func
+        self.name = func.__name__
+        if doc is not None:
+            self.__doc__ = doc
+        else:
+            self.__doc__ = func.__doc__
+        return self
+
+    def __get__(self, obj, typ):
+        cdef:
+            bint immutable
+
+        if obj is None:
+            # accessed on the class, not the instance
+            return self
+
+        # Default to non-caching
+        immutable = getattr(typ, '_immutable', False)
+        if not immutable:
+            # behave like a property
+            val = self.func(obj)
+            return val
+
+        # Get the cache or set a default one if needed
+        cache = getattr(obj, '_cache', None)
+        if cache is None:
+            try:
+                cache = obj._cache = {}
+            except AttributeError:
+                return
+
+        if PyDict_Contains(cache, self.name):
+            # not necessary to Py_INCREF
+            val = <object> PyDict_GetItem(cache, self.name)
+        else:
+            val = self.func(obj)
+            PyDict_SetItem(cache, self.name, val)
+        return val
+
+    def __set__(self, obj, value):
+        raise Exception("cannot set values for [%s]" % self.name)
+
+
 cdef class AxisProperty(object):
     cdef:
         Py_ssize_t axis

pandas/core/series.py

@@ -145,6 +145,8 @@ class Series(base.IndexOpsMixin, generic.NDFrame):
         ['asobject', 'sortlevel', 'reshape', 'get_value', 'set_value',
          'from_csv', 'valid'])
 
+    hasnans = property(base.IndexOpsMixin.hasnans.func)
+
     def __init__(self, data=None, index=None, dtype=None, name=None,
                  copy=False, fastpath=False):
 

pandas/tests/series/test_internals.py

@@ -8,6 +8,7 @@
 from numpy import nan
 import numpy as np
 
+import pandas as pd
 from pandas import Series
 from pandas.core.indexes.datetimes import Timestamp
 import pandas._libs.lib as lib
@@ -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 'enables' in pd.Series.hasnans.__doc__

pandas/tests/test_lib.py

@@ -5,6 +5,7 @@
 import numpy as np
 from pandas._libs import lib, writers as libwriters
 import pandas.util.testing as tm
+import pandas as pd
 
 
 class TestMisc(object):
@@ -198,3 +199,10 @@ def test_get_reverse_indexer(self):
         result = lib.get_reverse_indexer(indexer, 5)
         expected = np.array([4, 2, 3, 6, 7], dtype=np.int64)
         tm.assert_numpy_array_equal(result, expected)
+
+
+def test_cache_readonly_preserve_docstrings():
+    # GH#19700 accessing class attributes that are cache_readonly
+    # should a) not return None and b) reflect original docstrings
+    assert "perf" in pd.Index.hasnans.__doc__
+    assert "dtype str " in pd.Index.dtype_str.__doc__