Python-list Digest, Vol 27, Issue 123

Michael Williams mwilliams at mgreg.com
Fri Dec 9 01:14:35 CET 2005


Thanks, Heiko, I'll give this a try.  In the meantime, I'll try to  
explain what exactly I mean.

Basically, I want the ability to reference a variable just as I am  
able to set a variable (or attribute) on the fly.  For instance, say  
the user has the following list in a text file:

	[butter, cream, eggs, toast, jam]

I want to be able to loop through that and say the following:

	item.__setattr__(list[0].value,myclass())

At that point I have  item.butter, but I don't want to have to know  
(or hardcode) this, I want to then be able to do the following:

	item.__getattr__(list[0].value).__setattr__(list[1].value)

So that I now have:

	item.butter.cream

. . .and so on and so forth.  I want the entire thing to be  
dynamically created no matter what the user has entered.  And I for  
my part will simply limit the users input with a list of my own like so:

if list[i].value not in my_list:
	raise MyError()


I don't doubt that that's probably more confusing, but that's what I  
want to be able to do.

Regards,
Michael


On Dec 7, 2005, at 1:45 AM, python-list-request at python.org wrote:

> It's not exactly clear what you're trying to tell us here.  
> Basically, what I
> guess you want is:
>
> getattr(object,"varname")
>
> This retrieves the value that is bound to varname at "namespace"  
> object. You
> cannot retrieve a variable per se in Python, as a variable is just  
> a name
> that is a binding to an object.
>
> The second part of what you're trying to do sounds more like stacking
> objects. I'll just give it a shot and implement a little bit to  
> test on.
> You'll have to extend it to suit your needs... ;-)
>
> class xmlnode(object):
>
>     def __init__(self):
>         self.__subnodes = {}
>         self.__value = ""
>
>     def __getattr__(self,item):
>         if item == "value":
>             return self.__value
>         elif item.startswith("_"):
>             raise AttributeError, item
>         elif item not in self.__subnodes:
>             self.__subnodes[item] = xmlnode()
>         return self.__subnodes[item]
>
>     def __setattr__(self,item,value):
>         if item.startswith("_"):
>             super(xmlnode,self).__setattr__(item,value)
>         elif item == "value":
>             assert isinstance(value,(str,unicode))
>             self.__value = value
>         else:
>             assert isinstance(value,(str,unicode))
>             if item not in self.__subnodes:
>                 self.__subnodes[item] = xmlnode()
>             self.__subnodes[item].value = value
>
>     def __delattr__(self,item):
>         if item.startswith("_"):
>             super(xmlnode,self).__delattr__(item)
>         elif item == "value":
>             self.__value = None
>         else:
>             try:
>                 del self.__subnodes[item]
>             except KeyError:
>                 raise AttributeError, item
>
>     def toXML(self,name):
>         rv = ["<%s>" % name]
>         for sname, sitem in self.__subnodes.iteritems():
>             rv.append(sitem.toXML(sname))
>         if self.__value is not None:
>             rv.append(self.__value)
>         rv.append("</%s>" % name)
>         return "".join(rv)
>
> This implements a simple XML-tree builder with a very specific  
> output format
> (tags contain only one string value, which always comes after any  
> subtag
> that they might contain, and subtag order is random). The "tricky"  
> part is
> in the __*attr__ logic to get subnodes.
>
> Example of usage:
>
>>>> a = test.xmlnode()
>>>> a
> <test.xmlnode object at 0x2aaaaab57890>
>>>> a.value = "test"
>>>> a.toXML("root")
> '<root>test</root>'
>>>> a.book = "welcome"
>>>> a.anotherbook = "bye"
>>>> a.toXML("root")
> '<root><anotherbook>bye</anotherbook><book>welcome</book>test</root>'
>>>> a.book.value
> 'welcome'
>>>> a.book.anotherbook.somethingelse.thisislong = "test"
>>>> a.toXML("root")
> '<root><anotherbook>bye</ 
> anotherbook><book><anotherbook><somethingelse><thisislong>test</ 
> thisislong></somethingelse></anotherbook>welcome</book>test</root>'
>>>>
>
> HTH!
>
> --- Heiko.
>
>

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