[GitHub] [arrow] jorisvandenbossche commented on issue #10935: Readback KeyValueMetadata from Field and Schema in pyarrow from file written in C++

[GitBox <gi...@apache.org>]

jorisvandenbossche commented on issue #10935:
URL: https://github.com/apache/arrow/issues/10935#issuecomment-903601675


   When doing this in pure python, the field metadata gets preserved:
   
   ```python
   import pyarrow as pa
   import pyarrow.parquet as pq
   
   f = pa.field("a", pa.int64(), metadata={'test': 'value'})
   schema = pa.schema([f])
   table = pa.table({'a': [1, 2, 3]}, schema=schema)
   
   pq.write_table(table, "test_field_metadata.parquet")
   table2 = pq.read_table("test_field_metadata.parquet")
   
   # original table has the field metadata
   >>> table.field('a').metadata
   {b'test': b'value'}
   
   # roundtripped table still has it
   >>> table2.field('a').metadata
   {b'test': b'value'}
   ```
   
   So there is something that is done differently in your C++ example. I think this has to do with the saving of the Arrow schema in the Parquet FileMetaData or not. This is an option in the `ArrowWriterProperties`. In C++ it is set to False by default:
   
   https://github.com/apache/arrow/blob/b220fea6900e964f8fab987e7dacd6ae38c15dca/cpp/src/parquet/properties.h#L661
   
   but in Python we always enable it:
   
   https://github.com/apache/arrow/blob/b220fea6900e964f8fab987e7dacd6ae38c15dca/python/pyarrow/_parquet.pyx#L1294-L1296
   
   You can pass ArrowWriterProperties to the `parquet::arrow::WriteTable` function to enable store_schema in your example.


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