CLN: tighten what we pass to Index._shallow_copy (#31371)

Author: jbrockmendel

pandas/core/indexes/base.py

@@ -507,11 +507,6 @@ def _shallow_copy(self, values=None, **kwargs):
 
         attributes = self._get_attributes_dict()
         attributes.update(kwargs)
-        if not len(values) and "dtype" not in kwargs:
-            attributes["dtype"] = self.dtype
-
-        # _simple_new expects the type of self._data
-        values = getattr(values, "_values", values)
 
         return self._simple_new(values, **attributes)
 
@@ -2354,6 +2349,9 @@ def _get_unique_index(self, dropna: bool = False):
 
         if not self.is_unique:
             values = self.unique()
+            if not isinstance(self, ABCMultiIndex):
+                # extract an array to pass to _shallow_copy
+                values = values._data
 
         if dropna:
             try:

pandas/core/indexes/datetimes.py

@@ -377,7 +377,7 @@ def union_many(self, others):
 
     def _wrap_setop_result(self, other, result):
         name = get_op_result_name(self, other)
-        return self._shallow_copy(result, name=name, freq=None, tz=self.tz)
+        return self._shallow_copy(result, name=name, freq=None)
 
     # --------------------------------------------------------------------