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[jira] [Created] (ARROW-4088) Table.from_batches() fails when passed a schema with metadata


Thomas Buhrmann created ARROW-4088:
--------------------------------------

             Summary: Table.from_batches() fails when passed a schema with metadata
                 Key: ARROW-4088
                 URL: https://issues.apache.org/jira/browse/ARROW-4088
             Project: Apache Arrow
          Issue Type: Bug
          Components: C++, Python
    Affects Versions: 0.11.0
            Reporter: Thomas Buhrmann


This seems to be a regression. In 0.10 I used to have this function to set column-level and table-level metadata on an existing Table:
  
{code:python}
def set_metadata(tbl, col_meta={}, tbl_meta={}):
    # Create updated column fields with new metadata
    if col_meta or tbl_meta:
        fields = []
        for col in tbl.itercolumns():
            if col.name in col_meta:
                # Get updated column metadata
                metadata = col.field.metadata or {}
                for k, v in col_meta[col.name].items():
                    metadata[k] = json.dumps(v).encode('utf-8')
                # Update field with updated metadata
                fields.append(col.field.add_metadata(metadata))
            else:
                fields.append(col.field)

        # Get updated table metadata
        tbl_metadata = tbl.schema.metadata
        for k, v in tbl_meta.items():
            tbl_metadata[k] = json.dumps(v).encode('utf-8')

        # Create new schema with updated metadata
        schema = pa.schema(fields, metadata=tbl_metadata)

        # With updated schema build new table (shouldn't copy data?)
        tbl = pa.Table.from_batches(tbl.to_batches(), schema=schema)

    return tbl
{code}

However, in 0.11 this fails with error:

{noformat}
ArrowInvalid: Schema at index 0 was different: 
x: int64
vs
x: int64
...
{noformat}

It works however if I replace from_batches() with from_arrays(), like this:
{code}
tbl = pa.Table.from_arrays(list(tbl.itercolumns()), schema=schema)
{code}

It seems that from_batches() compares the existing batch's schema with the new schema, and upon encountering a difference (in metadata only) fails.

A short test would be this:
{code}
import pandas as pd
import pyarrow as pa

df = pd.DataFrame({'x': [0,1,2]})
tbl = pa.Table.from_pandas(df, preserve_index=False)

field = tbl.schema[0].add_metadata({'test': 'data'})
schema = pa.schema([field])
# tbl2 = pa.Table.from_arrays(list(tbl.itercolumns()), schema=schema)
tbl2 = pa.Table.from_batches(tbl.to_batches(), schema)
tbl2.schema[0].metadata
{code}



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