[Python] Add unnest/flatten function for List types

[asfimport]

Reporter: Wes McKinney / @wesm
Assignee: Krisztian Szucs / @kszucs

PRs and other links:

• GitHub Pull Request #2757

_{Note: This issue was originally created as ARROW-45. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Should this happen on the C++ side as well?

[asfimport]

Wes McKinney / @wesm:
Yes

[asfimport]

Antoine Pitrou / @pitrou:
Let's say we have the following array:

arr = pa.array([[1, None, 2], None, [3, 4]])

What should the output of flattening be?

• pa.array([1, None, 2, None, 3, 4])

• or pa.array([1, None, 2, 3, 4])

  The former would need a copy, I'm unsure about the latter (but it seems a null list entry could still span a non-empty child range depending on the offsets array).

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 2757
#2757