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BUG: Bug fix for implicit upcast to float64 for large series (more than 1000000 rows) #62077

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BUG: Bug fix for implicit upcast to float64 for large series (more than 1000000 rows) #62077

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b24df58
BUG: Bug fix for implicit upcast to float64 for large series
allecole Aug 8, 2025
e115956
Added test for float32 upcast in array_ops
allecole Aug 8, 2025
8c76040
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Aug 8, 2025
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1 change: 1 addition & 0 deletions doc/source/whatsnew/v3.0.0.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -752,6 +752,7 @@ Conversion
- Bug in :meth:`Series.astype` might modify read-only array inplace when casting to a string dtype (:issue:`57212`)
- Bug in :meth:`Series.convert_dtypes` and :meth:`DataFrame.convert_dtypes` removing timezone information for objects with :class:`ArrowDtype` (:issue:`60237`)
- Bug in :meth:`Series.reindex` not maintaining ``float32`` type when a ``reindex`` introduces a missing value (:issue:`45857`)
- Bug in :meth:`_array_ops._maybe_prepare_scalar_for_op` not maintaining ``float32`` type when converting NumPy floating scalars (:issue:`61951`)

Strings
^^^^^^^
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2 changes: 1 addition & 1 deletion pandas/core/ops/array_ops.py
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Expand Up @@ -573,7 +573,7 @@ def maybe_prepare_scalar_for_op(obj, shape: Shape):
return int(obj)

elif isinstance(obj, np.floating):
return float(obj)
return np.dtype(obj.dtype).type(obj)

return obj

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6 changes: 6 additions & 0 deletions pandas/tests/series/methods/test_convert_dtypes.py
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Expand Up @@ -318,3 +318,9 @@ def test_convert_dtype_pyarrow_timezone_preserve(self):
result = ser.convert_dtypes(dtype_backend="pyarrow")
expected = ser.copy()
tm.assert_series_equal(result, expected)

def test_float32_series_addition_preserves_dtype(self):
# GH#61951
ser_a = pd.Series(np.zeros(1000000), dtype="float32") + np.float32(1)
ser_b = pd.Series(np.zeros(1000001), dtype="float32") + np.float32(1)
assert all(dtype == np.float32 for dtype in (ser_a.dtype, ser_b.dtype))
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