ARROW-44: Python: prototype object model for array slot values ("scalars")

[wesm]

Non-exhaustive, but this will facilitate inspecting Arrow data while the library is in development.

In [2]: arr = arrow.from_pylist([['foo', None], None, [], ['qux']])

In [3]: arr
Out[3]: <arrow.array.ListArray at 0x7f1970030f98>

In [4]: arr[0]
Out[4]: ['foo', None]

In [5]: type(arr[0])
Out[5]: arrow.scalar.ListValue

In [6]: arr[0][0]
Out[6]: 'foo'

In [7]: arr[0][1]
Out[7]: NA

In [8]: arr[1]
Out[8]: NA

In [9]: arr[2]
Out[9]: []

In [10]: len(arr[2])
Out[10]: 0

In [11]: arr.type
Out[11]: DataType(list<string>)

[wesm]

And before you ask, it's turtles all the way down:

In [16]: arr = arrow.from_pylist([[[['foo']]]])

In [17]: arr.type
Out[17]: DataType(list<list<list<string>>>)

In [18]: arr[0].value_type
Out[18]: DataType(list<list<string>>)

In [19]: arr[0][0].value_type
Out[19]: DataType(list<string>)

In [20]: arr[0][0][0]
Out[20]: ['foo']

In [21]: arr[0][0][0][0]
Out[21]: 'foo'