[Python-Dev] PEP 246, redux (original) (raw)

Ian Bicking ianb at colorstudy.com
Wed Jan 12 23:07:37 CET 2005

Phillip J. Eby wrote:

Anyway, I'm honestly curious as to whether anybody can find a real situation where transitive adapter composition is an actual problem, as opposed to a theoretical one. I've heard a lot of people talk about what a bad idea it is, but I haven't heard any of them say they actually tried it. Conversely, I've also heard from people who have tried it, and liked it. However, at this point I have no way to know if this dichotomy is just a reflection of the fact that people who don't like the idea don't try it, and the people who either like the idea or don't care are open to trying it.

I haven't read through the entire thread yet, so forgive me if I'm redundant.

One case occurred to me with the discussion of strings and files, i.e., adapting from a string to a file. Let's say an IReadableFile, since files are too ambiguous.

Consider the case where we are using a path object, like Jason Orendorff's or py.path. It seems quite reasonable and unambiguous that a string could be adapted to such a path object. It also seems quite reasonable and unambiguous that a path object could be adapted to a IReadableFile by opening the file at the given path. It's also quite unambiguous that a string could be adapted to a StringIO object, though I'm not sure it's reasonable. In fact, it seems like an annoying but entirely possible case that some library would register such an adapter, and mess things up globally for everyone who didn't want such an adaptation to occur! But that's an aside. The problem is with the first example, where two seemingly innocuous adapters (string->path, path->IReadableFile) allow a new adaptation that could cause all sorts of problems (string->IReadableFile).

Ideally, if I had code that was looking for a file object and I wanted to accept filenames, I'd want to try to adapt to file, and if that failed I'd try to adapt to the path object and then from there to the file object. Or if I wanted it to take strings (that represented content) or file-like objects, I'd adapt to a file object and if that failed I'd adapt to a string, then convert to a StringIO object. A two-step adaptation encodes specific intention that it seems transitive adaption would be blind to.

As I think these things through, I'm realizing that registered adaptators really should be 100% accurate (i.e., no information loss, complete substitutability), because a registered adapter that seems pragmatically useful in one place could mess up unrelated code, since registered adapters have global effects. Perhaps transitivity seems dangerous because that has the potential to dramatically increase the global effects of those registered adapters.

-- Ian Bicking / ianb at colorstudy.com / http://blog.ianbicking.org

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