BUG: Series.__delitem__ converting EAs to ndarrays

doc/source/whatsnew/v1.3.0.rst

@@ -642,6 +642,7 @@ Indexing
 - Bug in setting ``numpy.timedelta64`` values into an object-dtype :class:`Series` using a boolean indexer (:issue:`39488`)
 - Bug in setting numeric values into a into a boolean-dtypes :class:`Series` using ``at`` or ``iat`` failing to cast to object-dtype (:issue:`39582`)
 - Bug in :meth:`DataFrame.loc.__setitem__` when setting-with-expansion incorrectly raising when the index in the expanding axis contains duplicates (:issue:`40096`)
+- Bug in :meth:`Series.__delitem__` with ``ExtensionDtype`` incorrectly casting to ``ndarray`` (:issue:`40386`)
 
 Missing
 ^^^^^^^

pandas/core/internals/blocks.py

@@ -1338,7 +1338,28 @@ def quantile(
         return new_block(result, placement=self._mgr_locs, ndim=2)
 
 
-class ExtensionBlock(Block):
+class EABackedBlock(Block):
+    """
+    Mixin for Block subclasses backed by ExtensionArray.
+    """
+
+    values: ExtensionArray
+
+    def delete(self, loc) -> None:
+        """
+        Delete given loc(-s) from block in-place.
+        """
+        # This will be unnecessary if/when __array_function__ is implemented
+        self.values = self.values.delete(loc)
+        self.mgr_locs = self._mgr_locs.delete(loc)
+        try:
+            self._cache.clear()
+        except AttributeError:
+            # _cache not yet initialized
+            pass
+
+
+class ExtensionBlock(EABackedBlock):
     """
     Block for holding extension types.
 
@@ -1647,7 +1668,7 @@ class NumericBlock(Block):
     is_numeric = True
 
 
-class NDArrayBackedExtensionBlock(Block):
+class NDArrayBackedExtensionBlock(EABackedBlock):
     """
     Block backed by an NDArrayBackedExtensionArray
     """
@@ -1754,19 +1775,6 @@ def fillna(
         new_values = values.fillna(value=value, limit=limit)
         return [self.make_block_same_class(values=new_values)]
 
-    def delete(self, loc) -> None:
-        """
-        Delete given loc(-s) from block in-place.
-        """
-        # This will be unnecessary if/when __array_function__ is implemented
-        self.values = self.values.delete(loc, axis=0)
-        self.mgr_locs = self._mgr_locs.delete(loc)
-        try:
-            self._cache.clear()
-        except AttributeError:
-            # _cache not yet initialized
-            pass
-
 
 class DatetimeLikeBlock(NDArrayBackedExtensionBlock):
     """Mixin class for DatetimeLikeBlock, DatetimeTZBlock."""

pandas/tests/series/indexing/test_delitem.py

@@ -3,6 +3,7 @@
 from pandas import (
     Index,
     Series,
+    date_range,
 )
 import pandas._testing as tm
 
@@ -50,3 +51,23 @@ def test_delitem_missing_key(self):
 
         with pytest.raises(KeyError, match=r"^0$"):
             del s[0]
+
+    def test_delitem_extension_dtype(self):
+        # GH#40386
+        # DatetimeTZDtype
+        dti = date_range("2016-01-01", periods=3, tz="US/Pacific")
+        ser = Series(dti)
+
+        expected = ser[[0, 2]]
+        del ser[1]
+        assert ser.dtype == dti.dtype
+        tm.assert_series_equal(ser, expected)
+
+        # PeriodDtype
+        pi = dti.tz_localize(None).to_period("D")
+        ser = Series(pi)
+
+        expected = ser[:2]
+        del ser[2]
+        assert ser.dtype == pi.dtype
+        tm.assert_series_equal(ser, expected)