[Python] Support protocols to extract Arrow objects from third-party classes

[asfimport]

In the style of NumPy's __array__, we should be able to ask inputs to pa.array, pa.Table.from_X, ... whether they can convert themselves to Arrow objects. This would allow for example to turn objects that hold an Arrow object internally to expose them directly instead of going a conversion path.

My current use case involves Pandas ExtensionArray instances that internally have Arrow objects and should be reused when we pass the whole DataFrame to pa.Table.from_pandas.

Reporter: Uwe Korn / @xhochy
Assignee: Joris Van den Bossche / @jorisvandenbossche

Related issues:

• [Python] support pandas' nullable Integer type in from_pandas (blocks)
• [Python] Interface for converting pandas ExtensionArray / other custom array objects to pyarrow Array (duplicates)

PRs and other links:

• GitHub Pull Request #5106

_{Note: This issue was originally created as ARROW-3829. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
I'm moving this to 0.13. If you submit an alpha / experimental version of this for 0.12, please go ahead =)

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 5106
#5106