From 82366678e79b6f3c15c016e783e2dfda6efd146b Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Fri, 2 Apr 2021 15:12:56 -0700
Subject: [PATCH 1/2] BUG: Series.__delitem__ converting EAs to ndarrays

---
 doc/source/whatsnew/v1.3.0.rst               |  1 +
 pandas/core/internals/blocks.py              | 36 ++++++++++++--------
 pandas/tests/series/indexing/test_delitem.py | 21 ++++++++++++
 3 files changed, 43 insertions(+), 15 deletions(-)

diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
index 75b2dee4a5822..49ddeb23e3193 100644
--- a/doc/source/whatsnew/v1.3.0.rst
+++ b/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
 ^^^^^^^
diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index 7c2a31e63eeb3..2eb0d90d5b730 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -1338,7 +1338,26 @@ def quantile(
         return new_block(result, placement=self._mgr_locs, ndim=2)
 
 
-class ExtensionBlock(Block):
+class EABackedBlockMixin:
+    """
+    Mixin for Block subclasses backed by 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(EABackedBlockMixin, Block):
     """
     Block for holding extension types.
 
@@ -1647,7 +1666,7 @@ class NumericBlock(Block):
     is_numeric = True
 
 
-class NDArrayBackedExtensionBlock(Block):
+class NDArrayBackedExtensionBlock(EABackedBlockMixin, Block):
     """
     Block backed by an NDArrayBackedExtensionArray
     """
@@ -1754,19 +1773,6 @@ 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."""
diff --git a/pandas/tests/series/indexing/test_delitem.py b/pandas/tests/series/indexing/test_delitem.py
index 019cb92d780ef..af6b3910baec0 100644
--- a/pandas/tests/series/indexing/test_delitem.py
+++ b/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)

From 0e15c58d02a17bbbad0834db6a262c169622132e Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Fri, 2 Apr 2021 17:40:40 -0700
Subject: [PATCH 2/2] mypy fixup

---
 pandas/core/internals/blocks.py | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index 2eb0d90d5b730..9a2b3be4b66e2 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -1338,11 +1338,13 @@ def quantile(
         return new_block(result, placement=self._mgr_locs, ndim=2)
 
 
-class EABackedBlockMixin:
+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.
@@ -1357,7 +1359,7 @@ def delete(self, loc) -> None:
             pass
 
 
-class ExtensionBlock(EABackedBlockMixin, Block):
+class ExtensionBlock(EABackedBlock):
     """
     Block for holding extension types.
 
@@ -1666,7 +1668,7 @@ class NumericBlock(Block):
     is_numeric = True
 
 
-class NDArrayBackedExtensionBlock(EABackedBlockMixin, Block):
+class NDArrayBackedExtensionBlock(EABackedBlock):
     """
     Block backed by an NDArrayBackedExtensionArray
     """