BUG: Don't cast categorical nan to int

doc/source/whatsnew/v1.0.0.rst

@@ -110,6 +110,7 @@ Categorical
 ^^^^^^^^^^^
 
 - Added test to assert the :func:`fillna` raises the correct ValueError message when the value isn't a value from categories (:issue:`13628`)
+- Bug in :meth:`Categorical.astype` where ``NaN`` values were handled incorrectly when casting to int (:issue:`28406`)
 -
 -
 

pandas/core/arrays/categorical.py

@@ -520,6 +520,9 @@ def astype(self, dtype: Dtype, copy: bool = True) -> ArrayLike:
             if dtype == self.dtype:
                 return self
             return self._set_dtype(dtype)
+        if is_integer_dtype(dtype) and self.isna().any():
+            msg = "Cannot cast to int."
+            raise ValueError(msg)
         return np.array(self, dtype=dtype, copy=copy)
 
     @cache_readonly

pandas/core/indexes/base.py

@@ -4713,13 +4713,13 @@ def set_value(self, arr, key, value):
     @Appender(_index_shared_docs["get_indexer_non_unique"] % _index_doc_kwargs)
     def get_indexer_non_unique(self, target):
         target = ensure_index(target)
-        if is_categorical(target):
-            target = target.astype(target.dtype.categories.dtype)
         pself, ptarget = self._maybe_promote(target)
         if pself is not self or ptarget is not target:
             return pself.get_indexer_non_unique(ptarget)
 
-        if self.is_all_dates:
+        if is_categorical(target):
+            tgt_values = np.asarray(target)
+        elif self.is_all_dates:
             tgt_values = target.asi8
         else:
             tgt_values = target._ndarray_values
@@ -4731,7 +4731,7 @@ def get_indexer_for(self, target, **kwargs):
         """
         Guaranteed return of an indexer even when non-unique.
 
-        This dispatches to get_indexer or get_indexer_nonunique
+        This dispatches to get_indexer or get_indexer_non_unique
         as appropriate.
 
         Returns

pandas/tests/extension/test_categorical.py

@@ -19,7 +19,7 @@
 import pytest
 
 import pandas as pd
-from pandas import Categorical
+from pandas import Categorical, CategoricalIndex
 from pandas.api.types import CategoricalDtype
 from pandas.tests.extension import base
 import pandas.util.testing as tm
@@ -197,7 +197,13 @@ def test_searchsorted(self, data_for_sorting):
 
 
 class TestCasting(base.BaseCastingTests):
-    pass
+    @pytest.mark.parametrize("cls", [Categorical, CategoricalIndex])
+    def test_cast_nan_to_int(self, cls):
+        # GH 28406
+        s = cls([0, 1, np.nan])
+
+        with pytest.raises((ValueError, TypeError), match="Cannot cast"):
+            s.astype(int)
 
 
 class TestArithmeticOps(base.BaseArithmeticOpsTests):