ARF created ARROW-6486:
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Summary: [Python] Allow subclassing & monkey-patching of Table
Key: ARROW-6486
URL: https://issues.apache.org/jira/browse/ARROW-6486
Project: Apache Arrow
Issue Type: Improvement
Components: Python
Reporter: ARF
Currently, many classes in ``pyarrow`` behave strangely to the Python user:
they are neither subclassable not monkey-patchable.
{{>>> import pyarrow as pa}}
{{>>> class MyTable(pa.Table):}}
{{... pass}}
{{...}}
{{>>> table = MyTable.from_arrays([], [])}}
{{>>> type(table)}}
{{<class 'pyarrow.lib.Table'>}}
The factory method did not return an instance of our subclass...
Never mind, let's monkey-patch {{Table}}:
{{}}
{{>>> pa.TableOriginal = pa.Table}}
{{>>> pa.Table = MyTable}}
{{>>> table = pa.Table.from_arrays([], [])}}
{{>>> type(table)}}
{{<class 'pyarrow.lib.Table'>}}
{{}}
OK, that did not work either.
Let's be sneaky:
{{>>> table.__class__ = MyTable}}
{{Traceback (most recent call last):}}
{{ File "<stdin>", line 1, in <module>}}
{{TypeError: __class__ assignment only supported for heap types or ModuleType
subclasses}}
{{>>>}}
There is currently no way to modify or extend the behaviour of a {{Table}}
instance. Users can use only what {{pyarrow}} provides out of the box. - This
is likely to be a source of frustration for many python users.
The attached PR remedies this for the {{Table}} class:
{{>>> import pyarrow as pa}}
{{>>> class MyTable(pa.Table):}}
{{... pass}}
{{...}}
{{>>> table = MyTable.from_arrays([], [])}}
{{>>> type(table)}}
{{<class '__main__.MyTable'>}}
{{>>>}}
{{>>> pa.TableOriginal = pa.Table}}
{{>>> pa.Table = MyTable}}
{{>>> table = pa.Table.from_arrays([], [])}}
{{>>> type(table)}}
{{<class '__main__.MyTable'>}}
{{>>>}}
Ideally, these modifications would be extended to the other cython-defined
classes of {{pyarrow}}, but given that Table is likely to be the interface that
most users begin their interaction with, I thought this would be a good start.
Keeping the changes limited to a single class should also keep merge conflicts
manageable.
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