From d4fdb66e32f841829ac0677ba87cceba2bb0f055 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Fri, 24 Dec 2021 15:05:20 -0800
Subject: [PATCH 1/2] BUG: Series[object].fillna ignoring downcast='infer'

---
 doc/source/whatsnew/v1.4.0.rst             |  2 ++
 pandas/core/generic.py                     |  5 +++
 pandas/core/internals/blocks.py            |  8 ++---
 pandas/tests/series/methods/test_fillna.py | 36 ++++++++++++++++++++++
 4 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/doc/source/whatsnew/v1.4.0.rst b/doc/source/whatsnew/v1.4.0.rst
index ccad93d83eb5b..a2a5660a3404e 100644
--- a/doc/source/whatsnew/v1.4.0.rst
+++ b/doc/source/whatsnew/v1.4.0.rst
@@ -732,6 +732,8 @@ Missing
 - Bug in :meth:`Series.interpolate` and :meth:`DataFrame.interpolate` with ``inplace=True`` not writing to the underlying array(s) in-place (:issue:`44749`)
 - Bug in :meth:`Index.fillna` incorrectly returning an un-filled :class:`Index` when NA values are present and ``downcast`` argument is specified. This now raises ``NotImplementedError`` instead; do not pass ``downcast`` argument (:issue:`44873`)
 - Bug in :meth:`DataFrame.dropna` changing :class:`Index` even if no entries were dropped (:issue:`41965`)
+- Bug in :meth:`Series.fillna` with an object-dtype incorrectly ignoring ``downcast="infer"`` (:issue:`44241`)
+-
 
 MultiIndex
 ^^^^^^^^^^
diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index fc15c846b1907..c5f461cadb517 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -6399,6 +6399,11 @@ def fillna(
         else:
             if self.ndim == 1:
                 if isinstance(value, (dict, ABCSeries)):
+                    if not len(value):
+                        # test_fillna_nonscalar
+                        if inplace:
+                            return
+                        return self.copy()
                     value = create_series_with_explicit_dtype(
                         value, dtype_if_empty=object
                     )
diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index 3f50cb2c601b5..9558b82d95fde 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -518,11 +518,11 @@ def split_and_operate(self, func, *args, **kwargs) -> list[Block]:
     def _maybe_downcast(self, blocks: list[Block], downcast=None) -> list[Block]:
 
         if self.dtype == _dtype_obj:
-            # TODO: why is behavior different for object dtype?
-            if downcast is not None:
-                return blocks
-
+            # GH#44241 We downcast regardless of the argument;
+            #  respecting 'downcast=None' may be worthwhile at some point,
+            #  but ATM it breaks too much existing code.
             # split and convert the blocks
+
             return extend_blocks(
                 [blk.convert(datetime=True, numeric=False) for blk in blocks]
             )
diff --git a/pandas/tests/series/methods/test_fillna.py b/pandas/tests/series/methods/test_fillna.py
index 160eb4774d1a0..19f61a0a2d6fc 100644
--- a/pandas/tests/series/methods/test_fillna.py
+++ b/pandas/tests/series/methods/test_fillna.py
@@ -189,6 +189,42 @@ def test_fillna_downcast(self):
         expected = Series([1, 0])
         tm.assert_series_equal(result, expected)
 
+    def test_fillna_downcast_infer_objects_to_numeric(self):
+        # GH#44241 if we have object-dtype, 'downcast="infer"' should
+        #  _actually_ infer
+
+        arr = np.arange(5).astype(object)
+        arr[3] = np.nan
+
+        ser = Series(arr)
+
+        res = ser.fillna(3, downcast="infer")
+        expected = Series(np.arange(5), dtype=np.int64)
+        tm.assert_series_equal(res, expected)
+
+        res = ser.ffill(downcast="infer")
+        expected = Series([0, 1, 2, 2, 4], dtype=np.int64)
+        tm.assert_series_equal(res, expected)
+
+        res = ser.bfill(downcast="infer")
+        expected = Series([0, 1, 2, 4, 4], dtype=np.int64)
+        tm.assert_series_equal(res, expected)
+
+        # with a non-round float present, we will downcast to float64
+        ser[2] = 2.5
+
+        expected = Series([0, 1, 2.5, 3, 4], dtype=np.float64)
+        res = ser.fillna(3, downcast="infer")
+        tm.assert_series_equal(res, expected)
+
+        res = ser.ffill(downcast="infer")
+        expected = Series([0, 1, 2.5, 2.5, 4], dtype=np.float64)
+        tm.assert_series_equal(res, expected)
+
+        res = ser.bfill(downcast="infer")
+        expected = Series([0, 1, 2.5, 4, 4], dtype=np.float64)
+        tm.assert_series_equal(res, expected)
+
     def test_timedelta_fillna(self, frame_or_series):
         # GH#3371
         ser = Series(

From 5c26503b7cc6b578e2aab0891ed8a5cd061ba347 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Fri, 24 Dec 2021 17:54:02 -0800
Subject: [PATCH 2/2] mypy fixup

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

diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index c5f461cadb517..7efbb299727e8 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -6402,7 +6402,7 @@ def fillna(
                     if not len(value):
                         # test_fillna_nonscalar
                         if inplace:
-                            return
+                            return None
                         return self.copy()
                     value = create_series_with_explicit_dtype(
                         value, dtype_if_empty=object