BUG: avoid validation error for ufunc with string[python] array #62498

Original file line number	Diff line number	Diff line change
Expand Up		@@ -48,6 +48,7 @@ Bug fixes
		with a compiled regex and custom flags (:issue:`62240`)
		- Fix :meth:`Series.str.match` and :meth:`Series.str.fullmatch` not matching patterns with groups correctly for the Arrow-backed string dtype (:issue:`61072`)
		- Fix comparing a :class:`StringDtype` Series with mixed objects raising an error (:issue:`60228`)
	- Fix error being raised when using a numpy ufunc with a Python-backed string array (:issue:`40800`)

		Improvements and fixes for Copy-on-Write
		~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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12 changes: 12 additions & 0 deletions pandas/core/arrays/numpy_.py

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -5,6 +5,7 @@
		Any,
		Literal,
		Self,
	cast,
		)

		import numpy as np
Expand Down Expand Up		@@ -48,6 +49,7 @@
		)

		from pandas import Index
	from pandas.arrays import StringArray


		class NumpyExtensionArray(
Expand Down Expand Up		@@ -234,6 +236,16 @@ def __array_ufunc__(self, ufunc: np.ufunc, method: str, inputs, *kwargs):
		# e.g. test_np_max_nested_tuples
		return result
		else:
	if self.dtype.type is str: # type: ignore[comparison-overlap]
Comment on lines 238 to +239 Copy link Member @mroeschke mroeschke Sep 29, 2025 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others. Learn more. nit: Would be a nice if this was `elif` so it could be un-indented one level, but not a big deal.
	# StringDtype
	self = cast("StringArray", self)
	try:
	# specify dtype to preserve storage/na_value
	return type(self)(result, dtype=self.dtype)
	except ValueError:
	# if validation of input fails (no strings)
	# -> fallback to returning raw numpy array
	return result
		# one return value; re-box array-like results
		return type(self)(result)

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27 changes: 27 additions & 0 deletions pandas/tests/arrays/string_/test_string.py

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Original file line number	Diff line number	Diff line change
Expand Up		@@ -840,3 +840,30 @@ def test_string_array_view_type_error():
		arr = pd.array(["a", "b", "c"], dtype="string")
		with pytest.raises(TypeError, match="Cannot change data-type for string array."):
		arr.view("i8")


	@pytest.mark.parametrize("box", [pd.Series, pd.array])
	def test_numpy_array_ufunc(dtype, box):
	arr = box(["a", "bb", "ccc"], dtype=dtype)

	# custom ufunc that works with string (object) input -> returning numeric
	str_len_ufunc = np.frompyfunc(lambda x: len(x), 1, 1)
	result = str_len_ufunc(arr)
	expected_cls = pd.Series if box is pd.Series else np.array
	# TODO we should infer int64 dtype here?
	expected = expected_cls([1, 2, 3], dtype=object)
	tm.assert_equal(result, expected)

	# custom ufunc returning strings
	str_multiply_ufunc = np.frompyfunc(lambda x: x * 2, 1, 1)
	result = str_multiply_ufunc(arr)
	expected = box(["aa", "bbbb", "cccccc"], dtype=dtype)
	if dtype.storage == "pyarrow":
	# TODO ArrowStringArray should also preserve the class / dtype
	if box is pd.array:
	expected = np.array(["aa", "bbbb", "cccccc"], dtype=object)
	else:
	# not specifying the dtype because the exact dtype is not yet preserved
	expected = pd.Series(["aa", "bbbb", "cccccc"])

	tm.assert_equal(result, expected)

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