> > So after all, what is a 'factory' or 'factory function'? > > A brief explanation in Python terms is at > http://www.aleax.it/ep03_pydp.pdf -- "pages" (slides) 37-44 (the rest of > the presentation is about an even more fundamental design pattern, > "template method"). A far more extensive essay can be found, for > example, at > http://gsraj.tripod.com/design/creational/factory/factory.html -- it > names the pattern "factory method", not "factory function" ("method" is > probably a more widespread name for it) and presents examples in Java > and Corba IDL. > > Anyway, trying to summarize: > > - "what": any function or method or other callable whose task it > is to build and return new objects can be called "a factory"; > > - sometimes a factory may get away with returning an existing > object for recycling "as if" it was a new one, saving some > resources, and this is one advantage; > > - a factory may choose what exact type of object to build and return > based on arguments or other circumstances, and this is a second > advantage. > > "Program to an interface, not to an implementation" -- the key > underlying principle ot the gang of 4's "Design Patterns" book. > > If you build an object of a specific given type ("new Foo" in Java > or C++), you're hardwiring the choice of implementation (the > exact concrete type); delegating the building to a method or > function frees your application from such hardwiring and lets > it be concentrated (if needed at all) in one spot (the factory). > [[With the "registry" pattern you may even be able to remove > any hardwiring, but that's a pretty advanced and subtle idea]]. > > > Alex
Thank you very much, this helped a lot. Just one side note, the url http://www.aleax.it/ep03_pydp.pdf should be http://www.aleax.it/Python/ep03_pydp.pdf, I found your document there. Thanks again. -- http://mail.python.org/mailman/listinfo/python-list
