avoid loss of precision

pandas-dev · mroeschke · Mar 10, 2023 · Mar 4, 2023 · Mar 4, 2023 · Mar 4, 2023
commit f60a3d6ad4eeeff2f57a4c627503e4334e257b03
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`)
+- Bug in :meth:`Series.combine_first` converting ``int64`` dtype to ``float64`` and losing precision on very large integers (:issue:`51764`)
 -

 Sparse

diff --git a/pandas/core/series.py b/pandas/core/series.py
@@ -93,13 +93,11 @@
 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,
@@ -3268,18 +3266,22 @@ def combine_first(self, other) -> Series:
        falcon      NaN
        dtype: float64
        """
-        new_index = self.index.union(other.index)
-        this = self.reindex(new_index, copy=False)
-        other = other.reindex(new_index, copy=False)
-        if this.dtype.kind == "M" and other.dtype.kind != "M":
-            other = to_datetime(other)
+        from pandas.core.reshape.concat import concat

-        combined = this.where(notna(this), other)
+        new_index = self.index.union(other.index)

-        if not is_dtype_equal(combined.dtype, self.dtype):
-            dtype = find_common_type([self.dtype, other.dtype])
-            combined = combined.astype(dtype, copy=False)
+        this = self
+        null_mask = isna(this)
+        if null_mask.any():
+            drop = this.index[null_mask].intersection(other.index[notna(other)])
+            if len(drop):
+                this = this.drop(drop)

+        other = other.reindex(other.index.difference(this.index), copy=False)
+        if this.dtype.kind == "M" and other.dtype.kind != "M":
+            other = to_datetime(other)
+        combined = concat([this, other]).reindex(new_index, copy=False)
+        combined.name = self.name
        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
@@ -115,8 +115,8 @@ def test_combine_first_timezone_series_with_empty_series(self):

    def test_combine_first_preserves_dtype(self):
        # GH51764
-        s1 = Series([4, 5])
-        s2 = Series([6, 7, 8])
+        s1 = Series([np.iinfo(np.int64).min, np.iinfo(np.int64).max])
+        s2 = Series([1, 2, 3])
        result = s1.combine_first(s2)
-        expected = Series([4, 5, 8])
+        expected = Series([np.iinfo(np.int64).min, np.iinfo(np.int64).max, 3])
        tm.assert_series_equal(result, expected)