BUG: OverflowError when fillna on DataFrame with a pd.Timestamp (pandas-dev#61208)

Author: PedroM4rques

doc/source/whatsnew/v3.0.0.rst

@@ -660,6 +660,7 @@ Datetimelike
 - Bug in :meth:`to_datetime` reports incorrect index in case of any failure scenario. (:issue:`58298`)
 - Bug in :meth:`to_datetime` wrongly converts when ``arg`` is a ``np.datetime64`` object with unit of ``ps``. (:issue:`60341`)
 - Bug in setting scalar values with mismatched resolution into arrays with non-nanosecond ``datetime64``, ``timedelta64`` or :class:`DatetimeTZDtype` incorrectly truncating those scalars (:issue:`56410`)
+- Bug in :meth:`DataFrame.fillna` raising an `AssertionError` instead of `OutOfBoundsDatetime` when filling a `datetime64[ns]` column with an out-of-bounds timestamp (e.g., `'0001-01-01'`). Now correctly raises `OutOfBoundsDatetime`. (:issue:`61208`)
 
 Timedelta
 ^^^^^^^^^

pandas/core/internals/blocks.py

@@ -1746,7 +1746,7 @@ def putmask(self, mask, new) -> list[Block]:
         try:
             # Caller is responsible for ensuring matching lengths
             values._putmask(mask, new)
-        except OutOfBoundsDatetime as e:
+        except OutOfBoundsDatetime:
             raise
         except (TypeError, ValueError):
             if self.ndim == 1 or self.shape[0] == 1:

pandas/tests/frame/methods/test_fillna.py

@@ -1,6 +1,8 @@
 import numpy as np
 import pytest
 
+from pandas.errors import OutOfBoundsDatetime
+
 from pandas import (
     Categorical,
     DataFrame,
@@ -15,7 +17,6 @@
 )
 import pandas._testing as tm
 from pandas.tests.frame.common import _check_mixed_float
-from pandas.errors import OutOfBoundsDatetime
 
 
 class TestFillNA:
@@ -782,19 +783,15 @@ def test_fillna_with_none_object(test_frame, dtype):
     if test_frame:
         expected = expected.to_frame()
     tm.assert_equal(result, expected)
-    
-    
+
+
 def test_fillna_out_of_bounds_datetime():
     # GH#61208
-    df = DataFrame({
-        'datetime': date_range('1/1/2011', periods=3, freq='h'),
-        'value': [1, 2, 3]
-    })
+    df = DataFrame(
+        {"datetime": date_range("1/1/2011", periods=3, freq="h"), "value": [1, 2, 3]}
+    )
     df.iloc[0, 0] = None
 
-    msg="Cannot cast 0001-01-01 00:00:00 to unit='ns' without overflow"
-    with pytest.raises(
-        OutOfBoundsDatetime,
-        match=msg
-    ):
-        df.fillna(Timestamp('0001-01-01'), inplace=True)
+    msg = "Cannot cast 0001-01-01 00:00:00 to unit='ns' without overflow"
+    with pytest.raises(OutOfBoundsDatetime, match=msg):
+        df.fillna(Timestamp("0001-01-01"), inplace=True)