BUG/PERF: Series.combine_first converting int64 to float64

doc/source/whatsnew/v2.1.0.rst

@@ -193,7 +193,7 @@ Groupby/resample/rolling
 
 Reshaping
 ^^^^^^^^^
--
+- Bug in :meth:`Series.combine_first` converting ``int64`` dtype to ``float64`` (:issue:`51764`)
 -
 
 Sparse

pandas/core/series.py

@@ -93,11 +93,13 @@
 from pandas.core.dtypes.cast import (
     LossySetitemError,
     convert_dtypes,
+    find_common_type,
     maybe_box_native,
     maybe_cast_pointwise_result,
 )
 from pandas.core.dtypes.common import (
     is_dict_like,
+    is_dtype_equal,
     is_extension_array_dtype,
     is_integer,
     is_iterator,
@@ -3272,7 +3274,13 @@ def combine_first(self, other) -> Series:
         if this.dtype.kind == "M" and other.dtype.kind != "M":
             other = to_datetime(other)
 
-        return this.where(notna(this), other)
+        combined = this.where(notna(this), other)
+
+        if not is_dtype_equal(combined.dtype, self.dtype):
+            dtype = find_common_type([self.dtype, other.dtype])
+            combined = combined.astype(dtype, copy=False)
+
+        return combined
 
     def update(self, other: Series | Sequence | Mapping) -> None:
         """

pandas/tests/series/methods/test_combine_first.py

@@ -112,3 +112,11 @@ def test_combine_first_timezone_series_with_empty_series(self):
         s2 = Series(index=time_index)
         result = s1.combine_first(s2)
         tm.assert_series_equal(result, s1)
+
+    def test_combine_first_preserves_dtype(self):
+        # GH51764
+        s1 = Series([4, 5])
+        s2 = Series([6, 7, 8])
+        result = s1.combine_first(s2)
+        expected = Series([4, 5, 8])
+        tm.assert_series_equal(result, expected)