Add a feature to override file extension defining compression

[dmcguire81]

Problem description

Transparent compression/decompression is a killer feature that could be made more broadly applicable, if decoupled from the extension of the underlying "file". Tying application-handling behavior to "file" extension is not a universally portable idea (for example, on classic UNIX, a "shebang" line encodes the same information), and some decoupling already exists in the current ignore_ext flag to smart_open.open. This feature would complete the decoupling by allowing the extension to be effectively overridden to force a compression-handling behavior, by supplying a override_ext string to smart_open.open. At that point, users would be free to name "files" without extensions, and still have the ability to take advantage of the great feature.

Steps/code to reproduce the problem

N/A (not a defect), but any file path without an extension is not eligible for transparent compression/decompression, currently.

Versions

>>> import platform, sys, smart_open
>>> print(platform.platform())
Darwin-19.6.0-x86_64-i386-64bit
>>> print("Python", sys.version)
Python 3.7.8 (default, Jul 27 2020, 17:21:35) 
[Clang 11.0.3 (clang-1103.0.32.59)]
>>> print("smart_open", smart_open.__version__)
smart_open 5.0.0.dev0

Checklist

Before you create the issue, please make sure you have:

• Described the problem clearly
• Provided a minimal reproducible example, including any required data
• Provided the version numbers of the relevant software

[mpenkov]

I think the current way we're handling compression is not ideal. It isn't obvious what the ignore_ext flag is doing, and it has no effect on files without an extension, as you've mentioned in the ticket and the associated PR.

This has been bothering me for a while, and I've come up with two solutions:

1. An additional parameter. As I've mentioned in the PR, it's not a particularly good idea. We already have tons of parameters. Plus, explaining how this new parameter will work in tandem with the existing ignore_ext flag will also confuse people.
2. Replace ignore_ext parameter with something that's more flexible and easier to explain to people. Of course, this would be a backwards-incompatible change, so we'd have to make a new major release for it.

Unless we find other alternatives, 2 is the way to go, but it will take some time to get it right. OTOH, we'd replace ignore_ext with a parameter like "compression", which will be an enum-like variable with the following values:

• none: no compression at all, equivalent to the current ignore_ext=True
• extension: default behavior, infer compression from extension, equievalent to current ignore_ext=False
• sniff: sniff compression from the first bytes of the file (during reading only)
• gz: gzip
• zst: zstandard
• ... and others

[aperiodic]

2. Replace ignore_ext parameter with something that's more flexible and easier to explain to people. Of course, this would be a backwards-incompatible change, so we'd have to make a new major release for it.

I agree that this is the best option, but why does it have to be backwards-incompatible? After the compression parameter is introduced, couldn't the implementation translate ignore_ext = False to compression = from_extension and ignore_ext = True to compression = none while emitting a deprecation warning? This would allow the new parameter to be introduced and ignore_ext to be deprecated while maintaining backwards-compatibility, and without the explainability and interaction downsides of the first approach.

(If both ignore_ext and compression are passed, then I think raising an exception is the best behavior, but this doesn't break backwards-compatibility.)

[mpenkov]

Good point @aperiodic. We could introduce the new parameter without immediately removing the old one.

[dmcguire81]

Thanks @mpenkov. Any feedback on when a release might get cut?

[mpenkov]

Probably within the next month or so.