Class.__class__ magic trick help

[Massimo Di Pierro]

On Aug 21, 2:40 am, Oscar Benjamin <oscar.j.benja... at gmail.com> wrote:
> On Mon, 20 Aug 2012 21:17:15 -0700 (PDT), Massimo Di Pierro
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> <massimo.dipie... at gmail.com> wrote:
> > Consider this code:
> > class SlowStorage(dict):
> >     def __getattr__(self,key):
> >           return self[key]
> >     def __setattr__(self,key):
> >           self[key]=value
> > class FastStorage(dict):
> >     def __init__(self, __d__=None, **kwargs):
> >         self.update(__d__,**kwargs)
> >     def __getitem__(self,key):
> >         return self.__dict__.get(key,None)
> >     def __setitem__(self,key,value):
> >         self.__dict__[key] = value
> >     def __delitem__(self,key):
> >         delattr(self,key)
> >     def __copy__(self):
> >         return Storage(self)
> >     def __nonzero__(self):
> >         return len(self.__dict__)>0
> >     def pop(self,key,default=None):
> >         if key in self:
> >             default = getattr(self,key)
> >             delattr(self,key)
> >         return default
> >     def clear(self):
> >         self.__dict__.clear()
> >     def __repr__(self):
> >         return repr(self.__dict__)
> >     def keys(self):
> >         return self.__dict__.keys()
> >     def values(self):
> >         return self.__dict__.values()
> >     def items(self):
> >         return self.__dict__.items()
> >       def iterkeys(self):
> >         return self.__dict__.iterkeys()
> >     def itervalues(self):
> >         return self.__dict__.itervalues()
> >     def iteritems(self):
> >         return self.__dict__.iteritems()
> >     def viewkeys(self):
> >         return self.__dict__.viewkeys()
> >     def viewvalues(self):
> >         return self.__dict__.viewvalues()
> >     def viewitems(self):
> >         return self.__dict__.viewitems()
> >     def fromkeys(self,S,v=None):
> >         return self.__dict__.fromkeys(S,v)
> >     def setdefault(self, key, default=None):
> >         try:
> >             return getattr(self,key)
> >         except AttributeError:
> >             setattr(self,key,default)
> >             return default
> >     def clear(self):
> >         self.__dict__.clear()
> >     def len(self):
> >         return len(self.__dict__)
> >     def __iter__(self):
> >         return self.__dict__.__iter__()
> >     def has_key(self,key):
> >         return key in self.__dict__
> >     def __contains__(self,key):
> >         return key in self.__dict__
> >     def update(self,__d__=None,**kwargs):
> >         if __d__:
> >             for key in __d__:
> >                 kwargs[key] = __d__[key]
> >         self.__dict__.update(**kwargs)
> >     def get(self,key,default=None):
> >         return getattr(self,key) if key in self else default
> > >>> s=SlowStorage()
> > >>> a.x=1  ### (1)
> > >>> a.x    ### (2)
> > 1 # ok
> > >>> isinstance(a,dict)
> > True # ok
> > >>> print dict(a)
> > {'x':1} # ok (3)
>
> Try:
>
> >>> a.items()
>
> What does that show?
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> > >>> s=FastStorage()
> > >>> a.x=1  ### (4)
> > >>> a.x    ### (5)
> > 1 # ok
> > >>> isinstance(a,dict)
> > True # ok
> > >>> print dict(a)
> > {} # not ok (6)
> > Lines (4) and (5) are about 10x faster then lines (1) and (2). I
> like
> > FastStorage better but while (3) behaves ok, (6) does not behave as
> I
> > want.
> > I intuitively understand why FastStorage is cannot cast into dict
> > properly.
> > What I do not know is how to make it do the casting properly without
> > losing the 10x speedup of FastStorage over SlowStorage.
> > Any idea?
>
> I don't really understand what your trying to do but since you didn't
> add the __setattr__ method to FastStorage the item is not added to
> the dictionary when you do a.x = 1
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> Oscar

>>> a.items()
[('x',1')]

all the APIs work as expected except casting to dict.