BUG: CategoricalIndex.get_indexer issue with NaNs (pandas-dev#45361)

categorical_index_obj.get_indexer(target) yields incorrect results when
categorical_index_obj contains NaNs, and target does not. The reason
for this is that, if target contains elements which do not match any
category in categorical_index_obj, they are replaced by NaNs. In such
a situation, if categorical_index_obj also has NaNs, then the corresp
elements in target are mapped to an index which is not -1

eg:
ci = pd.CategoricalIndex([1, 2, np.nan, 3])
other = pd.Index([2, 3, 4])

ci.get_indexer(other)
In the implementation of get_indexer, other becomes [2, 3, NaN]
which is mapped to index 2, in ci

Update:
np.isnan(target) was breaking the existing codebase.
As a solution, I have enclosed this line in a try-except block

Author: Shashank-Shet

pandas/core/indexes/base.py

@@ -3699,7 +3699,10 @@ def get_indexer(
         # After _maybe_cast_listlike_indexer, target elements which do not
         # belong to some category are changed to NaNs
         # Mask to track actual NaN values compared to inserted NaN values
-        target_nans = np.isnan(target)
+        try:
+            target_nans = np.isnan(target)
+        except TypeError as e:
+            target_nans = False
         target = self._maybe_cast_listlike_indexer(target)
 
         self._check_indexing_method(method, limit, tolerance)