[Baypiggies] Tricky dictionary iteration problem

corey.coughlin at comcast.net corey.coughlin at comcast.net
Fri Jan 6 03:05:01 CET 2012

Thanks, that looks pretty good. It still fails doing a .copy(), so I'll have to rewrite that. And see what else I need to fix. 

As far as why I want to do it goes, well, I originally had an IndexedList class, which contained a list and a dictionary. All the objects in the list had a .reference string attribute, and it worked mostly like a list with an index on it. I wrote getitem to check the index to see if it were an integer or a string, and if it was a string use the dictionary reference for a fast look up of the object. The problem there is that it eats a lot of memory for big data sets, so I was looking at using an OrderedDict based object to emulate that behavior. Works pretty well, except that I need to change all my loops from: 
for obj in indexlist: 
for obj in indexlist.values(): 
and that's kind of a pain. The simpler way to pull it off is to not inherit from OrderedDict, but just have an object attribute that holds an ordered dict and add all the mapping and iteration function calls manually. That works, but I was looking for a simpler way to do it. Looks like either way, I'll have to rewrite some functions. Thanks for the help! 

------- Corey 

----- Original Message -----
From: "Robert Zuber" <rob at zuber.net> 
To: "corey coughlin" <corey.coughlin at comcast.net> 
Cc: baypiggies at python.org 
Sent: Thursday, January 5, 2012 5:22:14 PM 
Subject: Re: [Baypiggies] Tricky dictionary iteration problem 

using super to avoid the recursion seems to work: 

>>> class od2(collections.OrderedDict): 
... def __iter__(self): 
... return (v for v in super(collections.OrderedDict, self).itervalues()) 
>>> od = od2() 
>>> od['a'] = 'b' 
>>> od['c'] = 'd' 
>>> [v for v in od] 
['b', 'd'] 

On Jan 5, 2012, at 4:57 PM, corey.coughlin at comcast.net wrote: 

> Hi all, 
> Sorry to bug everybody, but I'm having some trouble with a somewhat tricky class I'm trying to make. I have a class that inherits from collections.OrderedDict, in an effort to create an ordered dictionary with some extra functions. Now what I'd like to do is to change the iteration so that when you iterate over it, it iterates over values instead of keys. This is turning out to be harder than I thought. First, I tried this: 
> class od2(collections.OrderedDict): 
> def __iter__(self): 
> return iter(self.values()) 
> Now the code I'm running creates a dict and tries to do a .copy on it, but I'd expect the problem to come up with most kind of dictionary access. The problem is that it goes into an infinite recursion with messages like this 
> File "...", line 57, in __iter__ 
> return iter(self.values()) 
> File ".../python/2.7.1/linux64/lib/python2.7/_abcoll.py" line 372 in values 
> return [self[key] for key in self] 
> It looks like the values() function uses the iterator to generate the values. OK, so maybe I'll try this: 
> class od2(collections.OrderedDict): 
> def __iter__(self): 
> values = [self[key] for key in self.keys()] 
> return iter(values) 
> that gives me messages like this: 
> File "...", line 56, in __iter__ 
> values = [self[key] for key in self.keys()] 
> File ".../python/2.7.1/linux64/lib/python2.7/_abcoll.py" line 366 in keys 
> return list(self) 
> and list() uses the iterator. Gah. So I'm out of ideas here, even if I dig into OrderedDict and find some way to pull the keys out, I'll probably have to rewrite a bunch of access functions for it to still work like a dictionary. Am I just tilting at windmills, or is doing something like this actually possible? Or am I just being dense, and there's a super obvious way to do this? Thanks for any help! 
> (Oh, and forgive me if there's a known fix for this, googling for anything along these lines just sent me to tons of tutorials about using the .values() function. No help there.) 
> ------- Corey 
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