From 95f1cb34c97c308b4f6fb9f9b1baa06bc54f02cc Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Fri, 1 Sep 2023 13:39:28 +0200
Subject: [PATCH 1/3] REGR: interpolate raising if fill_value is given

---
 doc/source/whatsnew/v2.1.1.rst                  | 1 +
 pandas/core/generic.py                          | 2 +-
 pandas/tests/series/methods/test_interpolate.py | 7 +++++++
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v2.1.1.rst b/doc/source/whatsnew/v2.1.1.rst
index b7e45d043d869..8a59c6137bd5c 100644
--- a/doc/source/whatsnew/v2.1.1.rst
+++ b/doc/source/whatsnew/v2.1.1.rst
@@ -15,6 +15,7 @@ Fixed regressions
 ~~~~~~~~~~~~~~~~~
 - Fixed regression in :func:`read_csv` when ``usecols`` is given and ``dtypes`` is a dict for ``engine="python"`` (:issue:`54868`)
 - Fixed regression in :meth:`DataFrame.__setitem__` raising ``AssertionError`` when setting a :class:`Series` with a partial :class:`MultiIndex` (:issue:`54875`)
+- Fixed regression in :meth:`Series.interpolate` raising when ``fill_value`` was given (:issue:`54920`)
 - Fixed regression when comparing a :class:`Series` with ``datetime64`` dtype with ``None`` (:issue:`54870`)
 
 .. ---------------------------------------------------------------------------
diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index b9407ebe6624a..15b9ff66a5218 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -8180,7 +8180,7 @@ def interpolate(
                         stacklevel=find_stack_level(),
                     )
 
-        if "fill_value" in kwargs:
+        if method in fillna_methods and "fill_value" in kwargs:
             raise ValueError(
                 "'fill_value' is not a valid keyword for "
                 f"{type(self).__name__}.interpolate"
diff --git a/pandas/tests/series/methods/test_interpolate.py b/pandas/tests/series/methods/test_interpolate.py
index 619690f400d98..a558187b18a64 100644
--- a/pandas/tests/series/methods/test_interpolate.py
+++ b/pandas/tests/series/methods/test_interpolate.py
@@ -858,3 +858,10 @@ def test_interpolate_asfreq_raises(self):
         with pytest.raises(ValueError, match=msg):
             with tm.assert_produces_warning(FutureWarning, match=msg2):
                 ser.interpolate(method="asfreq")
+
+    def test_interpolate_fill_value(self):
+        # GH#54920
+        ser = Series([np.nan, 0, 1, np.nan, 3, np.nan])
+        result = ser.interpolate(method="nearest", fill_value=0)
+        expected = Series([np.nan, 0, 1, 1, 3, 0])
+        tm.assert_series_equal(result, expected)

From ae41e5edabbb7a9afabe236ad9dc74632bd308de Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <patrick_hoefler@gmx.net>
Date: Sat, 2 Sep 2023 10:58:23 +0200
Subject: [PATCH 2/3] Update test and message

---
 pandas/core/generic.py                          | 3 ++-
 pandas/tests/series/methods/test_interpolate.py | 1 +
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/pandas/core/generic.py b/pandas/core/generic.py
index 15b9ff66a5218..f94b3f187daab 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -8183,7 +8183,8 @@ def interpolate(
         if method in fillna_methods and "fill_value" in kwargs:
             raise ValueError(
                 "'fill_value' is not a valid keyword for "
-                f"{type(self).__name__}.interpolate"
+                f"{type(self).__name__}.interpolate with method from"
+                f"{fillna_methods}"
             )
 
         if isinstance(obj.index, MultiIndex) and method != "linear":
diff --git a/pandas/tests/series/methods/test_interpolate.py b/pandas/tests/series/methods/test_interpolate.py
index a558187b18a64..549f429f09d35 100644
--- a/pandas/tests/series/methods/test_interpolate.py
+++ b/pandas/tests/series/methods/test_interpolate.py
@@ -861,6 +861,7 @@ def test_interpolate_asfreq_raises(self):
 
     def test_interpolate_fill_value(self):
         # GH#54920
+        pytest.importorskip("scipy")
         ser = Series([np.nan, 0, 1, np.nan, 3, np.nan])
         result = ser.interpolate(method="nearest", fill_value=0)
         expected = Series([np.nan, 0, 1, 1, 3, 0])

From 843beee19fddb548adace27144622b5c28034515 Mon Sep 17 00:00:00 2001
From: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>
Date: Tue, 5 Sep 2023 11:42:59 -0700
Subject: [PATCH 3/3] Update pandas/core/generic.py

---
 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 f94b3f187daab..68e15303b69c0 100644
--- a/pandas/core/generic.py
+++ b/pandas/core/generic.py
@@ -8183,7 +8183,7 @@ def interpolate(
         if method in fillna_methods and "fill_value" in kwargs:
             raise ValueError(
                 "'fill_value' is not a valid keyword for "
-                f"{type(self).__name__}.interpolate with method from"
+                f"{type(self).__name__}.interpolate with method from "
                 f"{fillna_methods}"
             )