[ARROW-16254] [Python] Scalar constructors should be callable

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: Python
• External issue URL: https://github.com/apache/arrow/issues/31648
• Assignee: Unassigned
• Reporter: Antoine Pitrou
• Created: 20/Apr/22 12:08
• Updated: 11/Jan/23 11:42

Description

Currently, to create a Scalar instance you must call the pa.scalar factory function. This works fine for the simplest types, but not for more elaborate ones.

For example, in C++ it is trivial to create a ListScalar from the embedded std::shared_ptr<Array>. But not in Python:

>>> arr = pa.array(["abc", "def", "ghi"])
>>> pa.ListScalar(arr)
Traceback (most recent call last):
  Input In [7] in <cell line: 1>
    pa.ListScalar(arr)
TypeError: __init__() takes exactly 0 positional arguments (1 given)

There is a clumsy workaround that creates an intermediate ListArray with trivial offsets:

>>> list_arr = pa.ListArray.from_arrays([0, len(arr)], arr)
>>> list_arr[0]
<pyarrow.ListScalar: ['abc', 'def', 'ghi']>

Issue Links

• is related to: ARROW-5295 [Python] accept pyarrow values / scalars in constructor functions ?