Factory classes (was: Re: [Tutor] Java and Python (was: Iterators))

[Danny Yoo]

On Tue, 3 Sep 2002, Erik Price wrote:

>
> On Tuesday, September 3, 2002, at 10:35  AM, alan.gauld@bt.com wrote:
>
> > Usiakly the way to deal with class explosions is to define a
> > function (or factory class) which takes the parameters in,
> > figures out which kind of class you really need and hands
> > you a pointer to it:
>
> What is a factory class?  I have seen this term before, but I'm not sure
> how it is different from any other class.  Don't all class definitions
> create instances for you (handing you a reference to the instance)?

Hi Erik,

The Portland Patterns Repository collects descriptions of these nebulous
design patterns.  I did a quick search and found:

    http://c2.com/cgi/wiki?ClassFactory


Admittedly, it's a little dense, and doesn't have enough Python.  *grin*
What may be even better for Python programmers might be Bruce Eckel's
"Thinking In Python", which is an online book on exploring design patterns
in Python:

    http://www.mindview.net/Books/TIPython

In particular, Chapter 5 in Eckel's book talks about factories, and looks
really nice.



There are a few examples of factories in Python's standard library.  One
notorious one includes the xml.sax.make_parser() function that
manufactures XML parser objects for us.  make_parser() is a "factory
function",

    http://c2.com/cgi/wiki?FactoryFunction
    http://www.python.org/doc/lib/module-xml.sax.html

that will try its best to give us back a parser object.  It's only purpose
in life is to pick among several potential choices and give us something
reasonable to work with.

(But by default, I think it's only choice is the 'pyexpat' parser based on
a C extension module.  What makes make_parser()  notorious is that, when
people install Python on their own systems, they often forget to install
expat, which means that make_parser() has no way of making an xml parser
object for us.)

The 'pyxml' project provides a few more implementations of XML parsers, as
well as the '4suite' XML modules.  As far as we're concerned, though,
xml.sax.make_parser() manufactures us an xml parser object, and we usually
shouldn't care exactly where it comes from or how it got constructed.


Hope this helps!