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[jira] [Created] (ARROW-3654) [Python] Column with CategoricalIndex fails to be read back


Armin Berres created ARROW-3654:
-----------------------------------

             Summary: [Python] Column with CategoricalIndex fails to be read back
                 Key: ARROW-3654
                 URL: https://issues.apache.org/jira/browse/ARROW-3654
             Project: Apache Arrow
          Issue Type: Bug
          Components: Python
    Affects Versions: 0.11.1
            Reporter: Armin Berres


When a column with a \{Categoricalndex} is written the data can never be read back.

 {code:python}
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')

pq.read_pandas('test.parquet').to_pandas()
{code}

Results in

{code}
KeyError                                  Traceback (most recent call last)
~/venv/mpptool/lib/python3.7/site-packages/pyarrow/pandas_compat.py in _pandas_type_to_numpy_type(pandas_type)
    676     try:
--> 677         return _pandas_logical_type_map[pandas_type]
    678     except KeyError:

KeyError: 'categorical'
{code}

The schema looks good:
{code}
column_indexes": [{"name": "c1", "field_name": "c1", "pandas_type": "categorical", "numpy_type": "int8", "metadata": {"num_categories": 2, "ordered": false}}]
{code}



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