[Python][Parquet] Writing a sparse union column raises ArrowNotImplementedError #53595

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

pyarrow.parquet.write_table refuses a table with a sparse union column, whether the union is at the top level or nested in a list or struct:

import io

import pyarrow as pa
import pyarrow.parquet as pq

union = pa.UnionArray.from_sparse(
    pa.array([0, 1], type=pa.int8()),
    [pa.array([1, None]), pa.array([None, "x"])],
    field_names=["a", "b"],
)
pq.write_table(pa.table({"u": union}), io.BytesIO())

pyarrow.lib.ArrowNotImplementedError: Unhandled type for Arrow to Parquet schema conversion: sparse_union<a: int64=0, b: string=1>

pa.ListArray.from_arrays([0, 2], union) and pa.StructArray.from_arrays([union], names=["f"]) raise the same error.

pyarrow 26.0.0 and 25.0.1, Python 3.14, macOS arm64.

#28288 reported the same error for dense unions and was closed as stale without a resolution.

Parquet has no union type, but a sparse union's children are each as long as the union, so it maps onto a group of its children plus a type-code column. The stored Arrow schema (ARROW:schema) could then restore the union on read, as it already restores duration written as INT64. If write support is out of scope, documenting the limitation would help.

This issue was drafted with an AI assistant, which also ran the reproducer on pyarrow 26.0.0 and 25.0.1. I reviewed it before filing.

Component(s)

Parquet, Python