[Python-Dev] PEP 408 -- Standard library preview package (original) (raw)

Steven D'Aprano steve at pearwood.info
Sat Jan 28 03:51:41 CET 2012

Michael Foord wrote:

On 27/01/2012 20:48, Steven D'Aprano wrote:

Eli Bendersky wrote:

try: from preview import thing except ImportError: import thing

So no need to target a very specific version of Python.

Yep, this is what I had in mind. And it appeared too trivial to place it in the PEP. Trivial and wrong. Since thing and preview.thing may have subtle, or major, API differences, how do you use it? No, potentially wrong in cases where the APIs are different. Even with the try...except ImportError dance around StringIO / cStringIO there are some API differences. But for a lot of use cases it works fine (simplejson and json aren't identical, but it works for most people).

Okay, granted, I accept your point.

But I think we need to distinguish between these cases.

In the case of StringIO and cStringIO, API compatibility is expected, and differences are either bugs or implementation differences that you shouldn't be relying on.

In the case of the typical[1] preview module, one of the motivations of adding it to preview is to test the existing API. We should expect changes, even if in practice often there won't be. We might hope for no API changes, but we should plan for the case where there will be.

And that rules out the "try import" dance for the typical preview module. There may be modules which graduate and keep the same API. In those cases, people will quickly work out the import dance on their own, it's a very common idiom.

But we shouldn't advertise it as the right way to deal with preview, since that implies the expectation of API stability, and we want to send the opposite message: preview is the last time the API can change without a big song and dance, so be prepared for it to change.

I'm with Nick on this one: if you're not prepared to change "from preview import module" to "import module" in your app, then you certainly won't be prepared to deal with the potential API changes and you aren't the target audience for preview.

[1] I am fully aware of the folly of referring to a "typical" example of something that doesn't exist yet

-- Steven

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