[ARROW-16287] PyArrow: RuntimeError: AppendRowGroups requires equal schemas when writing _metadata file

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 7.0.0
• Component/s: Parquet
• Environment: MacOS. Python 3.8.10. ; pyarrow: '7.0.0' ; pandas: '1.4.2' ; numpy: '1.22.3'
• External issue URL: https://github.com/apache/arrow/issues/31678
• Assignee: Unassigned
• Reporter: Kyle Barron
• Created: 22/Apr/22 17:16
• Updated: 11/Jan/23 11:43

Description

I'm trying to follow the example here: https://arrow.apache.org/docs/python/parquet.html#writing-metadata-and-common-medata-files to write an example partitioned dataset. But I'm consistently getting an error about non-equal schemas. Here's a mcve:

from pathlib import Path
import numpy as np
import pandas as pd
import pyarrow as pa
import pyarrow.parquet as pq
size = 100_000_000
partition_col = np.random.randint(0, 10, size)
values = np.random.rand(size)
table = pa.Table.from_pandas(
    pd.DataFrame({"partition_col": partition_col, "values": values})
)
metadata_collector = []
root_path = Path("random.parquet")
pq.write_to_dataset(
    table,
    root_path,
    partition_cols=["partition_col"],
    metadata_collector=metadata_collector,
)

Write the ``_common_metadata`` parquet file without row groups statistics
pq.write_metadata(table.schema, root_path / "_common_metadata")


Write the ``_metadata`` parquet file with row groups statistics of all files
pq.write_metadata(
    table.schema, root_path / "_metadata", metadata_collector=metadata_collector
) 

This raises the error

---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
Input In [92], in <cell line: 1>()
----> 1 pq.write_metadata(
      2     table.schema, root_path / "_metadata", metadata_collector=metadata_collector
      3 )
File ~/tmp/env/lib/python3.8/site-packages/pyarrow/parquet.py:2324, in write_metadata(schema, where, metadata_collector, **kwargs)
   2322 metadata = read_metadata(where)
   2323 for m in metadata_collector:
-> 2324     metadata.append_row_groups(m)
   2325 metadata.write_metadata_file(where)
File ~/tmp/env/lib/python3.8/site-packages/pyarrow/_parquet.pyx:628, in pyarrow._parquet.FileMetaData.append_row_groups()
RuntimeError: AppendRowGroups requires equal schemas. 

But all schemas in the `metadata_collector` list seem to be the same:

all(metadata_collector[0].schema == meta.schema for meta in metadata_collector)
# True