address comments

pandas/core/indexes/base.py

@@ -361,7 +361,8 @@ def _outer_indexer(
     _can_hold_strings: bool = True
 
     # Whether this index is a NumericIndex, but not a Int64Index, Float64Index,
-    # UInt64Index or RangeIndex. Needed for backwards compat. Remove in pandas 2.0.
+    # UInt64Index or RangeIndex. Needed for backwards compat. Remove this attribute and
+    # associated code in pandas 2.0.
     _is_backward_compat_public_numeric_index: bool = False
 
     _engine_type: type[libindex.IndexEngine] = libindex.ObjectEngine

pandas/core/indexes/category.py

@@ -282,7 +282,7 @@ def _is_dtype_compat(self, other) -> Categorical:
         return other
 
     @doc(Index.astype)
-    def astype(self, dtype, copy: bool = True) -> Index:
+    def astype(self, dtype: Dtype, copy: bool = True) -> Index:
         from pandas.core.api import NumericIndex
 
         dtype = pandas_dtype(dtype)

pandas/core/indexes/numeric.py

@@ -166,6 +166,10 @@ def _ensure_array(cls, data, dtype, copy: bool):
         dtype = cls._ensure_dtype(dtype)
 
         if copy or not is_dtype_equal(data.dtype, dtype):
+            # the try/except below is because it's difficult to predict the error
+            # and/or error message from different combinations of data and type.
+            # Efforts to avoid this try/except welcome.
+            # See https://github.com/pandas-dev/pandas/pull/41153#discussion_r676206222
             try:
                 subarr = np.array(data, dtype=dtype, copy=copy)
                 cls._validate_dtype(subarr.dtype)
@@ -247,6 +251,9 @@ def astype(self, dtype, copy=True):
                 else:
                     return NumericIndex(arr, name=self.name, dtype=dtype)
         elif self._is_backward_compat_public_numeric_index:
+            # this block is needed so e.g. NumericIndex[int8].astype("int32") returns
+            # NumericIndex[int32] and not Int64Index with dtype int64.
+            # When Int64Index etc. are removed from the code base, removed this also.
             if not is_extension_array_dtype(dtype) and is_numeric_dtype(dtype):
                 return self._constructor(self, dtype=dtype, copy=copy)