how to convert string to list or tuple

[Duncan Booth]

Ruud de Jong wrote:

> Steven Bethard schreef:
>> But unless the person eval-ing your code *only* writes immaculate
>> code I can see that you can probably screw them. ;)  I wonder why 
>> __subclasses__ isn't a restricted attribute...  Is it ever used for 
>> something that isn't evil? ;)
>> 
>> STeVe
> 
> Completely off topic, but I just cannot resist showing off.
> Some time ago I used __subclasses__ in a way that is not evil. I
> think. 
> 
> The details are described in the following thread:
> http://groups.google.nl/group/comp.lang.python/browse_thread/thread/5c1
> ccb986c66cdc1/ 
> 
> A summary: I used __subclasses__ to apply the Chain-of-Responsibility
> pattern to object creation. The code would appear to instantiate
> an object of the root of a class hierarchy, but the actual object
> that was created would be an instance of a subclass.
> 
> So to get back to your question: yes, there are non-evil
> uses for __subclasses__. Weird perhaps, but non-evil.
> Non-standard, sure . Too clever for my own good, very likely.

I've done almost exactly the same thing. The base class uses __subclasses__ 
to find the best matching subclass based on the factory parameters. In my 
case I was retrieving files from the web, so I had a base Handler class and 
created HtmlHandler, ImageHandler &c.

class Handler(object):
    '''Class to process files'''
    __map = {}

    @classmethod
    def _resolveClass(klass, isdir, name):
        map = Handler.__map
        if not map:
            for c in klass.__subclasses__():
                for ext in c.Extensions:
                    map['.'+ext.lower()] = c

        if isdir:
            klass = FolderHandler
        else:
            ext = os.path.splitext(name)[1].lower()
            if ext not in map:
                map[ext] = DefaultHandler

            klass = map[ext]
        return klass(name)
        
    @classmethod
    def fromPathname(klass, name, path, uri, db):
        isdir = os.path.isdir(os.path.join(path, name))
        obj = klass._resolveClass(isdir, name)
        obj._initialize(name, path, uri, db)
        return obj

    @classmethod
    def fromUrl(klass, uri, text, db=None):
    	   ... and so on ...

and then subclasses such as:

class ImageHandler(Handler):
    Extensions = ('jpg', 'jpeg', 'gif', 'png')
    type = 'Image'

class DefaultHandler(Handler):
    Extensions = ('',)
    type = 'Ignored'

This also contains the only code I think I've written with a class 
definition in a for loop:

# General categories
EXTENSIONS = {
    'js': 'javascript',
    'php': 'php',
    'doc': 'Word Document',
    'xls': 'Spreadsheet',
    'ppt': 'Powerpoint',
    'css': 'Stylesheet',
    'swf': 'Flash',
    'pdf': 'File',
    'rtf': 'File',
    'zip': 'File',
    }

Classes = []
for ext in EXTENSIONS:
    class GeneralHandler(Handler):
        Extensions = (ext,)
        type = EXTENSIONS[ext]

    Classes.append(GeneralHandler)