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[Python] CategoricalIndex is lost after reading back #19959

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Description

@asfimport

When a CategoricalIndex is written and read back the resulting index is not more categorical.

df = pd.DataFrame([['a', 'b'], ['c', 'd']], columns=['c1', 'c2'])
df['c1'] = df['c1'].astype('category')
df = df.set_index(['c1'])

table = pa.Table.from_pandas(df)
pq.write_table(table, 'test.parquet')

ref_df = pq.read_pandas('test.parquet').to_pandas()

print(df.index)
# CategoricalIndex(['a', 'c'], categories=['a', 'c'], ordered=False, name='c1', dtype='category')

print(ref_df.index)
# Index(['a', 'c'], dtype='object', name='c1')

In the metadata the information is correctly contained:

{"name": "c1", "field_name": "c1", "p'
            b'andas_type": "categorical", "numpy_type": "int8", "metadata": {"'
            b'num_categories": 2, "ordered": false}

 

Reporter: Armin Berres
Assignee: Wes McKinney / @wesm

Related issues:

PRs and other links:

Note: This issue was originally created as ARROW-3652. Please see the migration documentation for further details.

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