From 8923cc854c1b4a0f0cd14ed039efee18b2379257 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Mon, 18 Jan 2021 21:03:26 -0800
Subject: [PATCH 1/3] BUG: setting dt64 values into Series[int] incorrectly
 casting dt64->int

---
 pandas/core/internals/blocks.py              | 12 +++++++++++
 pandas/tests/series/indexing/test_setitem.py | 21 ++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index 1356b9d3b2ca3..4b645e548a58b 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -991,6 +991,18 @@ def setitem(self, indexer, value):
 
         # set
         else:
+            if (
+                self.is_object
+                and not is_ea_value
+                and arr_value.dtype.kind in ["m", "M"]
+            ):
+                # https://github.com/numpy/numpy/issues/12550
+                #  numpy will incorrect cast to int if we're not careful
+                if is_list_like(value):
+                    value = list(value)
+                else:
+                    value = [value] * len(values[indexer])
+
             values[indexer] = value
 
         if transpose:
diff --git a/pandas/tests/series/indexing/test_setitem.py b/pandas/tests/series/indexing/test_setitem.py
index 7f469f361fec7..c0a60d6d6a0d4 100644
--- a/pandas/tests/series/indexing/test_setitem.py
+++ b/pandas/tests/series/indexing/test_setitem.py
@@ -239,6 +239,27 @@ def test_setitem_callable_other(self):
         tm.assert_series_equal(ser, expected)
 
 
+class TestSetitemCasting:
+    @pytest.mark.parametrize("dtype", ["M8[ns]", "m8[ns]"])
+    def test_setitem_dt64_into_int_series(self, dtype):
+        # dont cast dt64 to int when doing this setitem
+        orig = Series([1, 2, 3])
+
+        val = np.datetime64("2021-01-18 13:25:00", "ns")
+        if dtype == "m8[ns]":
+            val = val - val
+
+        ser = orig.copy()
+        ser[:-1] = val
+        expected = Series([val, val, 3], dtype=object)
+        tm.assert_series_equal(ser, expected)
+        assert isinstance(ser[0], type(val))
+
+        ser = orig.copy()
+        ser[:-1] = np.array([val, val])
+        tm.assert_series_equal(ser, expected)
+
+
 @pytest.mark.parametrize(
     "obj,expected,key",
     [

From 1076052f7f62e0415ca3dbdda705d5f6c5aa2c3d Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Tue, 19 Jan 2021 12:18:00 -0800
Subject: [PATCH 2/3] Whatsnew

---
 doc/source/whatsnew/v1.3.0.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
index 6a85bfd852e19..24f31832be8ec 100644
--- a/doc/source/whatsnew/v1.3.0.rst
+++ b/doc/source/whatsnew/v1.3.0.rst
@@ -281,6 +281,7 @@ Indexing
 - Bug in :meth:`DataFrame.loc` dropping levels of :class:`MultiIndex` when :class:`DataFrame` used as input has only one row (:issue:`10521`)
 - Bug in setting ``timedelta64`` values into numeric :class:`Series` failing to cast to object dtype (:issue:`39086`)
 - Bug in setting :class:`Interval` values into a :class:`Series` or :class:`DataFrame` with mismatched :class:`IntervalDtype` incorrectly casting the new values to the existing dtype (:issue:`39120`)
+- Bug in setting ``datetime64`` values into a :class:`Series` with integer-dtype incorrect casting the datetime64 values to integers (:issue:`39266`)
 
 Missing
 ^^^^^^^

From 38b5646dda9fe820e33e0570b6473a763eb14217 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Wed, 20 Jan 2021 14:28:15 -0800
Subject: [PATCH 3/3] de-nest condition

---
 pandas/core/internals/blocks.py | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index 884161849e68c..758ee24543d1d 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -987,19 +987,17 @@ def setitem(self, indexer, value):
                 # TODO(EA2D): special case not needed with 2D EA
                 values[indexer] = value.to_numpy(values.dtype).reshape(-1, 1)
 
-        # set
+        elif self.is_object and not is_ea_value and arr_value.dtype.kind in ["m", "M"]:
+            # https://github.com/numpy/numpy/issues/12550
+            #  numpy will incorrect cast to int if we're not careful
+            if is_list_like(value):
+                value = list(value)
+            else:
+                value = [value] * len(values[indexer])
+
+            values[indexer] = value
+
         else:
-            if (
-                self.is_object
-                and not is_ea_value
-                and arr_value.dtype.kind in ["m", "M"]
-            ):
-                # https://github.com/numpy/numpy/issues/12550
-                #  numpy will incorrect cast to int if we're not careful
-                if is_list_like(value):
-                    value = list(value)
-                else:
-                    value = [value] * len(values[indexer])
 
             values[indexer] = value