[ARROW-7266] [Python] dictionary_encode() of a slice gives wrong result

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.15.1
• Fix Version/s: 0.16.0
• Component/s: C++, Python
• Labels: pull-request-available
• Environment: Docker on Linux 5.2.18-200.fc30.x86_64; Python 3.7.4
• External issue URL: https://github.com/apache/arrow/issues/23556
• Assignee: Antoine Pitrou
• Reporter: Adam Hooper
• Created: 26/Nov/19 14:34
• Updated: 11/Jan/23 07:52
• Resolved: 19/Dec/19 06:44

Description

Steps to reproduce:

import pyarrow as pa
arr = pa.array(["a", "b", "b", "b"])[1:]
arr.dictionary_encode()

Expected results:

-- dictionary:
  [
    "b"
  ]
-- indices:
  [
    0,
    0,
    0
  ]

Actual results:

-- dictionary:
  [
    "b",
    ""
  ]
-- indices:
  [
    0,
    0,
    1
  ]

I don't know a workaround. Converting to pylist and back is too slow. Is there a way to copy the slice to a new offset-0 StringArray that I could then dictionary-encode? Otherwise, I'm considering building buffers by hand....

Issue Links

• links to: GitHub Pull Request #6061