chrikrah opened a new issue, #55410:
URL: https://github.com/apache/arrow/issues/55410
### Describe the bug, including details regarding any error messages,
version, and platform.
The docstrings of `utf8_is_printable` and `ascii_is_printable` say an empty
string gives false. Both kernels return true for it.
```python
import pyarrow as pa
import pyarrow.compute as pc
print(pa.__version__)
a = pa.array(["", "a", None])
for fn in (pc.utf8_is_printable, pc.ascii_is_printable, pc.utf8_is_alpha):
print(fn.__name__, fn(a).to_pylist())
print(pc.utf8_is_printable.__doc__.splitlines()[2:4])
```
```
$ python -I pr.py # pyarrow 26.0.0 wheel, Python 3.14, Linux x86_64
26.0.0
utf8_is_printable [True, True, None]
ascii_is_printable [True, True, None]
utf8_is_alpha [False, True, None]
['For each string in `strings`, emit true iff the string is non-empty', 'and
consists only of printable Unicode characters. Null strings emit null.']
```
contract: the generated docstring above, from `StringClassifyDoc(...,
/*non_empty=*/true)` at
`cpp/src/arrow/compute/kernels/scalar_string_utf8.cc:313` and
`scalar_string_ascii.cc:671` (`aae29bf`). The user guide says the same for the
whole String predicates table, "An empty string element emits false in the
output" (`docs/source/cpp/compute.rst:905`), with no note on the two printable
rows.
The behaviour looks deliberate. Both kernels instantiate their predicate
with `/*allow_empty=*/true` (`scalar_string_utf8.cc:242`,
`scalar_string_ascii.cc:599`), the C++ tests assert `""` gives true
(`IsPrintableUnicode`, `IsPrintableAscii`), and `"".isprintable()` is `True` in
Python, the function `test_string_py_compat_boolean` compares against.
Expected: both docs pass `false` to `StringClassifyDoc`, giving "emit true
iff the string consists only of printable ... characters", and `compute.rst`
gets a note on `ascii_is_printable` and `utf8_is_printable` saying an empty
string emits true.
I read the source at `aae29bf` and ran the 26.0.0 wheel, not a source build.
I'll follow up with a PR that flips both arguments and adds the note in
`compute.rst`.
### Component(s)
C++, Documentation, Python
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