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https://issues.apache.org/jira/browse/ARROW-5139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16833597#comment-16833597
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Joris Van den Bossche edited comment on ARROW-5139 at 5/6/19 9:37 AM:
----------------------------------------------------------------------
[~fjetter] thanks for the report! A little bit simpler reproducible example,
without parquet (but the same underlying reason, the rangeindex is indeed not
constructed again for empty tables):
{code}
In [1]: import pyarrow as pa
In [2]: pa.__version__
Out[2]: '0.12.0'
In [3]: df = pd.DataFrame(
...: {"a": [1, 2]}
...: )
In [4]: table = pa.Table.from_pandas(df, columns=[], preserve_index=True)
In [5]: table
Out[5]:
pyarrow.Table
__index_level_0__: int64
metadata
--------
OrderedDict([(b'pandas',
b'{"index_columns": ["__index_level_0__"], "column_indexes": ['
b'{"name": null, "field_name": null, "pandas_type": "unicode",'
b' "numpy_type": "object", "metadata": {"encoding": "UTF-8"}}]'
b', "columns": [{"name": null, "field_name": "__index_level_0_'
b'_", "pandas_type": "int64", "numpy_type": "int64", "metadata'
b'": null}], "pandas_version": "0.23.4"}')])
In [6]: print(table.to_pandas())
Empty DataFrame
Columns: []
Index: [0, 1]
In [7]: table.to_pandas().index
Out[7]: Int64Index([0, 1], dtype='int64')
{code}
But the above, now gives:
{code}
In [4]: table
Out[4]:
pyarrow.Table
metadata
--------
{b'pandas': b'{"index_columns": [{"kind": "range", "name": null, "start": 0, "'
b'stop": 2, "step": 1}], "column_indexes": [{"name": null, "field_'
b'name": null, "pandas_type": "unicode", "numpy_type": "object", "'
b'metadata": {"encoding": "UTF-8"}}], "columns": [], "creator": {"'
b'library": "pyarrow", "version": "0.13.1.dev126+ga9ae4a9f.d201905'
b'03"}, "pandas_version": "0.24.2"}'}
In [5]: print(table.to_pandas())
Empty DataFrame
Columns: []
Index: []
In [6]: table.to_pandas().index
Out[6]: RangeIndex(start=0, stop=0, step=1)
{code}
was (Author: jorisvandenbossche):
[~fjetter] thanks for the report! A little bit easier reproducible example,
without parquet (but the same underlying reason, the rangeindex is indeed not
constructed again for empty tables):
{code}
In [1]: import pyarrow as pa
In [2]: pa.__version__
Out[2]: '0.12.0'
In [3]: df = pd.DataFrame(
...: {"a": [1, 2]}
...: )
In [4]: table = pa.Table.from_pandas(df, columns=[], preserve_index=True)
In [5]: table
Out[5]:
pyarrow.Table
__index_level_0__: int64
metadata
--------
OrderedDict([(b'pandas',
b'{"index_columns": ["__index_level_0__"], "column_indexes": ['
b'{"name": null, "field_name": null, "pandas_type": "unicode",'
b' "numpy_type": "object", "metadata": {"encoding": "UTF-8"}}]'
b', "columns": [{"name": null, "field_name": "__index_level_0_'
b'_", "pandas_type": "int64", "numpy_type": "int64", "metadata'
b'": null}], "pandas_version": "0.23.4"}')])
In [6]: print(table.to_pandas())
Empty DataFrame
Columns: []
Index: [0, 1]
In [7]: table.to_pandas().index
Out[7]: Int64Index([0, 1], dtype='int64')
{code}
But the above, now gives:
{code}
In [4]: table
Out[4]:
pyarrow.Table
metadata
--------
{b'pandas': b'{"index_columns": [{"kind": "range", "name": null, "start": 0, "'
b'stop": 2, "step": 1}], "column_indexes": [{"name": null, "field_'
b'name": null, "pandas_type": "unicode", "numpy_type": "object", "'
b'metadata": {"encoding": "UTF-8"}}], "columns": [], "creator": {"'
b'library": "pyarrow", "version": "0.13.1.dev126+ga9ae4a9f.d201905'
b'03"}, "pandas_version": "0.24.2"}'}
In [5]: print(table.to_pandas())
Empty DataFrame
Columns: []
Index: []
In [6]: table.to_pandas().index
Out[6]: RangeIndex(start=0, stop=0, step=1)
{code}
> [Python/C++] Empty column selection no longer restores index
> ------------------------------------------------------------
>
> Key: ARROW-5139
> URL: https://issues.apache.org/jira/browse/ARROW-5139
> Project: Apache Arrow
> Issue Type: Bug
> Components: C++, Python
> Affects Versions: 0.12.1
> Reporter: Florian Jetter
> Priority: Minor
> Labels: parquet
>
> The index of a dataframe is no longer reconstructed when using empty column
> selection. This is a regression to 0.12.1 and probably only happens for
> pd.RangeIndex
> {code:python}
> import pandas as pd
> import pyarrow as pa
> import pyarrow.parquet as pq
> from kartothek.serialization import ParquetSerializer
> from storefact import get_store_from_url
> print(pa.__version__)
> df = pd.DataFrame(
> {"a": [1, 2]}
> )
> print(df.index)
> table = pa.Table.from_pandas(df)
> buf = pa.BufferOutputStream()
> pq.write_table(table, buf)
> reader = pa.BufferReader(buf.getvalue().to_pybytes())
> table_restored = pq.read_pandas(reader, columns=[])
> df_restored = table_restored.to_pandas()
> print(len(df_restored))
> {code}
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