[jira] [Updated] (ARROW-12609) [Python] TypeError when accessing length of an invalid ListScalar

["Nic Crane (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-12609?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Nic Crane updated ARROW-12609:
------------------------------
    Summary: [Python] TypeError when accessing length of an invalid ListScalar  (was: TypeError when accessing length of an invalid ListScalar)

> [Python] TypeError when accessing length of an invalid ListScalar
> -----------------------------------------------------------------
>
>                 Key: ARROW-12609
>                 URL: https://issues.apache.org/jira/browse/ARROW-12609
>             Project: Apache Arrow
>          Issue Type: Bug
>          Components: Python
>    Affects Versions: 3.0.0, 4.0.0
>         Environment: Windows 10
> python=3.9.2
> pyarrow=4.0.0 (3.0.0 has the same behavior)
>            Reporter: Sergey Mozharov
>            Priority: Major
>
> For List-like data types, the scalar corresponding to a missing value has '___len___' attribute, but TypeError is raised when it is accessed
> {code:java}
> import pyarrow as pa
> data_type = pa.list_(pa.struct([
>     ('a', pa.int64()),
>     ('b', pa.bool_())
> ]))
> data = [[{'a': 1, 'b': False}, {'a': 2, 'b': True}], None]
> arr = pa.array(data, type=data_type)
> missing_scalar = arr[1]  # <pyarrow.ListScalar: None>
> assert hasattr(missing_scalar, '__len__')
> assert len(missing_scalar) == 0  # --> TypeError: object of type 'NoneType' has no len()
> {code}
> Expected behavior: length is expected to be 0.
> This issue causes several pandas unit tests to fail when an ExtensionArray backed by arrow array with this data type is built.
> This behavior is also inconsistent with a similar example where the data type is a struct:
> {code:java}
> import pyarrow as pa
> data_type = pa.struct([
>     ('a', pa.int64()),
>     ('b', pa.bool_())
> ])
> data = [{'a': 1, 'b': False}, None]
> arr = pa.array(data, type=data_type)
> missing_scalar = arr[1]  # <pyarrow.StructScalar: None>
> assert hasattr(missing_scalar, '__len__')
> assert len(missing_scalar) == 0  # Ok
> {code}
>  In this second example the TypeError is not raised.



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