From d4c31fe9f53450015c21faf42b65ca2e348248eb Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Thu, 29 Sep 2022 18:01:15 +0200
Subject: [PATCH 1/3] REGR: replace replacing wrong values with inplace and
 datetime

---
 doc/source/whatsnew/v1.5.1.rst            |  1 +
 pandas/core/internals/blocks.py           |  5 ++++-
 pandas/tests/frame/methods/test_fillna.py | 13 +++++++++++++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.5.1.rst b/doc/source/whatsnew/v1.5.1.rst
index a8ec836ba93df..d6d7ed9ea4ad6 100644
--- a/doc/source/whatsnew/v1.5.1.rst
+++ b/doc/source/whatsnew/v1.5.1.rst
@@ -77,6 +77,7 @@ Fixed regressions
 - Fixed performance regression in :func:`factorize` when ``na_sentinel`` is not ``None`` and ``sort=False`` (:issue:`48620`)
 - Fixed regression causing an ``AttributeError`` during warning emitted if the provided table name in :meth:`DataFrame.to_sql` and the table name actually used in the database do not match (:issue:`48733`)
 - Fixed regression in :func:`to_datetime` when ``arg`` was a date string with nanosecond and ``format`` contained ``%f`` would raise a ``ValueError`` (:issue:`48767`)
+- Fixed regression in :meth:`DataFrame.fillna` replacing wrong values for ``datetime64[ns]`` dtype and ``inplace=True`` (:issue:`48863`)
 - Fixed :meth:`.DataFrameGroupBy.size` not returning a Series when ``axis=1`` (:issue:`48738`)
 - Fixed Regression in :meth:`DataFrameGroupBy.apply` when user defined function is called on an empty dataframe (:issue:`47985`)
 
diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index 62d8d7e504b4a..b45a6d818d4aa 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -1515,7 +1515,10 @@ def putmask(self, mask, new) -> list[Block]:
         """
         mask = extract_bool_array(mask)
 
-        values = self.values
+        if self.ndim == 2:
+            values = self.values.T
+        else:
+            values = self.values
 
         orig_new = new
         orig_mask = mask
diff --git a/pandas/tests/frame/methods/test_fillna.py b/pandas/tests/frame/methods/test_fillna.py
index 697b28a65ac2e..ccd564b46cffa 100644
--- a/pandas/tests/frame/methods/test_fillna.py
+++ b/pandas/tests/frame/methods/test_fillna.py
@@ -13,6 +13,7 @@
     TimedeltaIndex,
     Timestamp,
     date_range,
+    to_datetime,
 )
 import pandas._testing as tm
 from pandas.tests.frame.common import _check_mixed_float
@@ -682,6 +683,18 @@ def test_fillna_with_columns_and_limit(self):
         tm.assert_frame_equal(result, expected)
         tm.assert_frame_equal(result2, expected2)
 
+    def test_fillna_datetime_inplace(self):
+        # GH#48863
+        df = DataFrame(
+            {
+                "date1": to_datetime(["2018-05-30", None]),
+                "date2": to_datetime(["2018-09-30", None]),
+            }
+        )
+        expected = df.copy()
+        df.fillna(np.nan, inplace=True)
+        tm.assert_frame_equal(df, expected)
+
     def test_fillna_inplace_with_columns_limit_and_value(self):
         # GH40989
         df = DataFrame(

From f9f3a36ae2c8457f0a70b0492fc520e02996cd77 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Thu, 29 Sep 2022 19:28:04 +0200
Subject: [PATCH 2/3] Fix arrow

---
 pandas/core/internals/blocks.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index b45a6d818d4aa..3690ab07ee8c1 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -1515,7 +1515,7 @@ def putmask(self, mask, new) -> list[Block]:
         """
         mask = extract_bool_array(mask)
 
-        if self.ndim == 2:
+        if self.values.ndim == 2:
             values = self.values.T
         else:
             values = self.values

From 586cbf9edcad76c2ec6abc974688335093b82090 Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Thu, 29 Sep 2022 19:38:28 +0200
Subject: [PATCH 3/3] Refactor

---
 pandas/core/internals/blocks.py | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index 3690ab07ee8c1..afe18493ec276 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -1515,10 +1515,9 @@ def putmask(self, mask, new) -> list[Block]:
         """
         mask = extract_bool_array(mask)
 
-        if self.values.ndim == 2:
-            values = self.values.T
-        else:
-            values = self.values
+        values = self.values
+        if values.ndim == 2:
+            values = values.T
 
         orig_new = new
         orig_mask = mask