Series.combine_first to preserve dtype

pandas-dev · mroeschke · Mar 10, 2023 · Mar 4, 2023 · Mar 4, 2023 · Mar 4, 2023
commit d28c39c518f387383c1b166f15627c9b4025024f
diff --git a/doc/source/whatsnew/v2.1.0.rst b/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

diff --git a/pandas/core/series.py b/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:
        """

diff --git a/pandas/tests/series/methods/test_combine_first.py b/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)