AlenkaF opened a new issue, #34165:
URL: https://github.com/apache/arrow/issues/34165
### Describe the bug, including details regarding any error messages,
version, and platform.
When working on the extension type for tensors in PyArrow I came across a
behaviour of the conversion to pandas that could be improved.
Creating an extension array (fixed shape tensor in this case) and converting
it to pandas works well
```python
>>> arr = [[1, 2, 3, 4], [10, 20, 30, 40], [100, 200, 300, 400]]
>>> storage = pa.array(arr, pa.list_(pa.int32(), 4))
>>> tensor = pa.ExtensionArray.from_storage(tensor_type, storage)
>>> tensor.to_pandas()
0 [1, 2, 3, 4]
1 [10, 20, 30, 40]
2 [100, 200, 300, 400]
dtype: object
```
But creating a table with an extension array and then converting it to
pandas fails:
```python
>>> data = [
... pa.array([1, 2, 3]),
... pa.array(['foo', 'bar', None]),
... pa.array([True, None, True]),
... tensor
... ]
>>> my_schema = pa.schema([('f0', pa.int8()),
... ('f1', pa.string()),
... ('f2', pa.bool_()),
... ('tensors_int', tensor_type)])
>>> table = pa.Table.from_arrays(data, schema=my_schema)
>>> table.to_pandas()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "pyarrow/array.pxi", line 830, in
pyarrow.lib._PandasConvertible.to_pandas
return self._to_pandas(options, categories=categories,
File "pyarrow/table.pxi", line 4004, in pyarrow.lib.Table._to_pandas
mgr = table_to_blockmanager(
File "/Users/alenkafrim/repos/arrow/python/pyarrow/pandas_compat.py", line
820, in table_to_blockmanager
blocks = _table_to_blocks(options, table, categories, ext_columns_dtypes)
File "/Users/alenkafrim/repos/arrow/python/pyarrow/pandas_compat.py", line
1171, in _table_to_blocks
return [_reconstruct_block(item, columns, extension_columns)
File "/Users/alenkafrim/repos/arrow/python/pyarrow/pandas_compat.py", line
1171, in <listcomp>
return [_reconstruct_block(item, columns, extension_columns)
File "/Users/alenkafrim/repos/arrow/python/pyarrow/pandas_compat.py", line
776, in _reconstruct_block
pandas_dtype = extension_columns[name]
KeyError: 'tensors_int'
```
The issue is due to the extension array in this example not having
`to_pandas_dtype` method implemented. In this case `ext_columns` does not get
populated in `_get_extension_dtypes` method with the name of the column with an
extension type:
https://github.com/apache/arrow/blob/0368e410be4dac30eada13d307b415165aedc6a7/python/pyarrow/pandas_compat.py#L870-L879
It would be good if it would, in case `to_pandas_dtype` method is not
implemented, convert the storage array
https://github.com/apache/arrow/blob/0368e410be4dac30eada13d307b415165aedc6a7/python/pyarrow/pandas_compat.py#L776
similar to
https://github.com/apache/arrow/blob/925cbd81427ae02ce897c406a264d53c8813b920/python/pyarrow/array.pxi#L2888-L2889
### Component(s)
Python
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