[Python] show_info() reports one codec twice and omits lz4_raw

[Nishuuzz]

Describe the bug, including details regarding any error messages, version, and platform.

pa.show_info() prints its Compression Codecs section from a hardcoded list in python/pyarrow/__init__.py:

codecs = ["brotli", "bz2", "gzip", "lz4_frame", "lz4", "snappy", "zstd"]

That list has drifted from _ensure_compression() in io.pxi, which is what actually decides which codec names are accepted. Two things follow from it:

• lz4 and lz4_frame both map to CCompressionType_LZ4_FRAME, so the same codec is reported twice under two names.
• lz4_raw maps to CCompressionType_LZ4 and is a separate codec, but it is not listed at all, even though pa.Codec.is_available('lz4_raw') returns True.

So seven lines are printed for six codecs, and the codec that is missing is a real one.

Observed, on pyarrow 25.0.0:

>>> import pyarrow as pa
>>> pa.show_info()
...
Compression Codecs:
  brotli              : Enabled
  bz2                 : Enabled
  gzip                : Enabled
  lz4_frame           : Enabled
  lz4                 : Enabled
  snappy              : Enabled
  zstd                : Enabled

Expected: each supported codec listed once, including lz4_raw.

Walking every name _ensure_compression() accepts and the enum value each maps to:

bz2       -> BZ2
gzip      -> GZIP
brotli    -> BROTLI
lz4       -> LZ4_FRAME
lz4_frame -> LZ4_FRAME   # same codec as lz4
lz4_raw   -> LZ4         # distinct, not reported by show_info()
snappy    -> SNAPPY
zstd      -> ZSTD

Platform: Windows 11, CPython 3.12, pyarrow 25.0.0. The list is unchanged on current main, so it affects main too.

A fix is proposed in #51753.

Component(s)

Python