[Python] Interface for converting pandas ExtensionArray / other custom array objects to pyarrow Array

[asfimport]

Related to ARROW-2428, which describes the issue to convert back to an ExtensionArray in to_pandas.

To start supporting to convert custom ExtensionArrays (eg the nullable Int64Dtype in pandas, or the arrow-backed fletcher arrays, ...) to arrow Arrays (eg in pyarrow.array(..)), I think it would be good to define an interface or hook that external projects can implement and that pyarrow will call if available.
This would allow external projects to define how they can be converted to arrow arrays, without the need that pyarrow itself starts to gather a lot of special cased code for certain types (like pandas' nullable Int64).

This could similar to how numpy looks for the \_\_array\_\_ method, so we might call it \_\_arrow_array\_\_.

See also pandas-dev/pandas#20612 for an issue discussing this on the pandas side.

Reporter: Joris Van den Bossche / @jorisvandenbossche

Related issues:

• [Python] support pandas' nullable Integer type in from_pandas (blocks)
• [Python] Support protocols to extract Arrow objects from third-party classes (is duplicated by)
• [Python] Add API to map Arrow types (including extension types) to pandas ExtensionArray instances for to_pandas conversions (is related to)

_{Note: This issue was originally created as ARROW-5271. Please see the migration documentation for further details.}

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
I am going to close this issue in favor of the earlier identical issue ARROW-3829. And I also opened a PR for this: #5106