[Python-3000] Discussions with no PEPs (original) (raw)

Phillip J. Eby pje at telecommunity.com
Wed Mar 14 00:12:54 CET 2007

At 08:37 AM 3/14/2007 +1000, Nick Coghlan wrote:

Greg Ewing wrote: > Nick Coghlan wrote: > >> Generics can live quite happily inside modules and as methods on objects. > > If instead of pydoc it's some third party module, > the situation is even worse, since the module may > not even be installed on the user's system. > > So this scheme would seem to create undesirable > dependencies.

In cases where you don't already have such a dependency, yeah.

This is why the idea of lazy importing comes up pretty fast whenever generics are discussed.

I call 'em "weak" imports, myself:

http://peak.telecommunity.com/DevCenter/Importing#weak-imports

As it happens, you need the same thing for adaptation, which is why I put this into PEAK way back in the heydey of Python 2.2.

However, this criticism regarding importing otherwise unneeded modules also applies to interfaces and ABC's, only more so. For example, if I wanted to declare that a class implements (or inherits from) pydoc.IDocumentable, then I would have to import pydoc first, and as I need to do it at class definition time (assuming we're using the normal inheritance mechanism to declare our interfaces), lazy importing can't help.

3rd-party registration of 'A implements I' or 'here's how to convert A to something that implements I' is useful not only for adapting two libraries to play well together, but also in allowing the libraries themselves to delay these definitions until they're known to be at least potentially neeeded (e.g. don't register a class as being documentable until the application actually imports and starts using pydoc).

An excellent point, indeed.

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