Fix PeriodIndex._shallow_copy allowing object-dtype (#30713)

Author: jbrockmendel

pandas/core/indexes/api.py

@@ -198,6 +198,7 @@ def conv(i):
         result = indexes[0]
 
         if hasattr(result, "union_many"):
+            # DatetimeIndex
             return result.union_many(indexes[1:])
         else:
             for other in indexes[1:]:

pandas/core/indexes/base.py

@@ -1947,7 +1947,7 @@ def dropna(self, how="any"):
             raise ValueError(f"invalid how option: {how}")
 
         if self.hasnans:
-            return self._shallow_copy(self.values[~self._isnan])
+            return self._shallow_copy(self._values[~self._isnan])
         return self._shallow_copy()
 
     # --------------------------------------------------------------------
@@ -2568,11 +2568,11 @@ def symmetric_difference(self, other, result_name=None, sort=None):
         left_indexer = np.setdiff1d(
             np.arange(this.size), common_indexer, assume_unique=True
         )
-        left_diff = this.values.take(left_indexer)
+        left_diff = this._values.take(left_indexer)
 
         # {other} minus {this}
         right_indexer = (indexer == -1).nonzero()[0]
-        right_diff = other.values.take(right_indexer)
+        right_diff = other._values.take(right_indexer)
 
         the_diff = concat_compat([left_diff, right_diff])
         if sort is None:

pandas/core/indexes/period.py

@@ -272,19 +272,14 @@ def _shallow_copy(self, values=None, **kwargs):
             values = self._data
 
         if isinstance(values, type(self)):
-            values = values._values
+            values = values._data
 
         if not isinstance(values, PeriodArray):
-            if isinstance(values, np.ndarray) and is_integer_dtype(values.dtype):
+            if isinstance(values, np.ndarray) and values.dtype == "i8":
                 values = PeriodArray(values, freq=self.freq)
             else:
-                # in particular, I would like to avoid period_array here.
-                # Some people seem to be calling use with unexpected types
-                # Index.difference -> ndarray[Period]
-                # DatetimelikeIndexOpsMixin.repeat -> ndarray[ordinal]
-                # I think that once all of Datetime* are EAs, we can simplify
-                # this quite a bit.
-                values = period_array(values, freq=self.freq)
+                # GH#30713 this should never be reached
+                raise TypeError(type(values), getattr(values, "dtype", None))
 
         # We don't allow changing `freq` in _shallow_copy.
         validate_dtype_freq(self.dtype, kwargs.get("freq"))