I suppose subs should be changed to just iterate over the entries.
Another option would be to create an object that implements __iter__
that returns (old, new) pairs. Does subs do this just for simplicity,
or does it need to have all the pairs at once before it can do the
substitution?
Aaron Meurer
On Mon, Apr 29, 2013 at 8:01 PM, Alan Bromborsky <abro...@verizon.net> wrote:
> I wrote a class (see below) for a automatically updating dictionary.
>
> For example let -
>
> def xsq(x):
> return(x*x)
>
> D = auto_update_dict(xsq)
>
> D[2] calls xsq and returns 4
> call D[2] and xsq is not called since the previous call has called xsq and
> placed
> the result in the dictionary. Again 4 is returned.
>
> When I use auto_update_dict in subs it does not work since since if the
> argument
> in subs is a dict the first thing subs does is to save D.items() and it
> never works
> with D again so that the state of D.items() is frozen for the sub call and
> the D.items()
> and D is never updated during the call to sub.
>
> The purpose of the auto_update_dict is to insure that one never has to
> generate
> a dictionary entry more than once so that time consuming calls to complex
> functions are not repeated.
>
> Can anyone suggest a work around to the problem with subs and dictionaries?
>
> class auto_update_dict(dict):
> """
> auto_update_dict creats entries to a dictionary on the fly. When the
> dictionary is called and the key used is not one of the existing keys
> The function self.f_update(key) is called to evaluate the key. The
> result is then added to the dictionary so that self.f_update is not
> used to evaluate the same key again.
>
> The __init__ is used to input self.f_update for a given dictionary.
> """
> def __init__(self, f_update, instance=None):
> self.f_update = f_update
> self.instance = instance
> self._dict = {}
>
> def __getitem__(self, key):
> try:
> return dict.__getitem__(self, key)
> except KeyError:
> try:
> if self.instance is None:
> f_key = self.f_update(key)
> else:
> f_key = self.f_update(self.instance,key)
> self[key] = f_key
> dict.__setitem__(self, key, f_key)
> return dict.__getitem__(self, key)
> except ValueError:
> raise ValueError('"f_update(' + str(key) + ')" not defined '
> + 'for key')
>
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