[Python] pa.types.is_nested() returns False for run-end encoded types #50847

Describe the bug, including details regarding any error messages, version, and platform.

pyarrow.types.is_nested() reports that a run-end encoded type is not nested, but the C++ arrow::is_nested() says it is.

>>> import pyarrow as pa
>>> t = pa.run_end_encoded(pa.int32(), pa.string())
>>> t.num_fields
2
>>> pa.types.is_nested(t)
False

The predicate in Python is backed by a hardcoded _NESTED_TYPES set in python/pyarrow/types.py, and Type_RUN_END_ENCODED was never added to it. If you line that set up against is_nested() in cpp/src/arrow/type_traits.h, run-end encoded is the only type the two disagree about — everything else matches, including the list-view types.

It also disagrees with the type itself: run-end encoded has two children (run ends and values), and every other type in pyarrow with children answers True here.

The same thing happened with fixed-size list, reported in #40171 and fixed in #40172, and the list-view types were added later. This looks like the last one that got missed when run-end encoding was added.

Tested with pyarrow 25.0.0 on Windows, and the same _NESTED_TYPES set is still there on main.

Component(s)

Python