[Python] Expose compressed file readers as NativeFile

[asfimport]

Reporter: Wes McKinney / @wesm
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #2839

_{Note: This issue was originally created as ARROW-3576. Please see the migration documentation for further details.}

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Wes McKinney / @wesm:
e.g. similar to gzip.GzipFile in Python

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Antoine Pitrou / @pitrou:
NativeFile currently requires a RandomAccessFile, but compressed files are mere InputStream. Fixing this will require rethinking NativeFile semantics.

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Wes McKinney / @wesm:
Indeed, it would need to be modified to support file handles not supporting seek

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Antoine Pitrou / @pitrou:
What should the API be here? CompressedReader and CompressedWriter classes (or CompressedInputStream CompressedOutputStream) would mirror the C++ API and make the implementation easy.

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Wes McKinney / @wesm:
I was thinking we could change NativeFile to have a single shared_ptr to the base FileInterface and then cast pointers depending on the capabilities of the file.

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Antoine Pitrou / @pitrou:
That's a possible implementation, but I was asking more about the Python-visible API. Do we add a compression argument to the OSFile constructor? Something else?

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Wes McKinney / @wesm:
Oh I see. I think having CompressedReader that takes any readable NativeFile (and CompressedWriter + writable NativeFile) is a good strategy

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 2839
#2839