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[Python-ideas] Dict-like object with property access

Massimo Di Pierro massimo.dipierro at gmail.com
Mon Jan 30 20:05:26 CET 2012 
Let me add that one can do this:

class Dummy(object):
   def __getitem__(self,key): return getattr(self,key)
   def __setitem__(self,key,value): return setattr(self,key,value)
   def __getattr__(self,key): return self.__dict__.get(key,None)

d=Dummy()
d.something = 5
print d.something
print d.somethingelse

which is more or less efficient and accomplished 4. Not as fast as a dict anyway.

On Jan 30, 2012, at 12:49 PM, Massimo Di Pierro wrote:

> Trying to make sure I understand where we disagree and perhaps explain my problem better. For me this has very little to do with dictionaries.
> 
> STEP 1) Today we can do this:
> 
>    class Dummy(object): pass
> 
>    d = Dummy()
>    d.something = 5
>    print d.something
> 
> Is anybody calling this un-pythonic?
> 
> STEP 2) We can now overload the [] to make the dynamic attributes accessible in an alternative syntax:
> 
>    class Dummy(object):
>          def __getitem__(self,key): return getattr(self,key)
>          def __setitem__(self,key,value): return setattr(self,key,value)
>    d = Dummy()
>    d.something = 5
>    d['something'] = 5
>    print d.something
>    print d['something']
> 
> STEP 3) Is anybody calling this un-pythonic?
> 
> We can add redefine __getattr__ so that it never raises an exception:
> 
> STEP 4)
> class Dummy(object):
>          def __getitem__(self,key): return getattr(self,key)
>          def __setitem__(self,key,value): return setattr(self,key,value)
>          def __getattr__(self,key):
>                  return object.__getattr__(self,key) if hasattr(self,key) else Dummy()
> 
> Is this un-pythonic?
> 
> I do not think so but I do have a problem with it:
> 
>    class Dummy(object):
>        def __getitem__(self,key): return getattr(self,key)
>        def __setitem__(self,key,value): return setattr(self,key,value)
>        def __getattr__(self,key):
>            print 'wtf'
>            return object.__getattr__(self,key) if hasattr(self,key) else None
> 
>>>> d=Dummy()
>>>> print d.somethingelse
>    wtf
>    ... 334 wtf times with python2.7, 999 times with python2.5...
>    wtf
>    None
> 
> whatever this does internally, it makes some programs slower then I would like them to be. Why is it calling itself 334 times?
> 
> STEP 5) 
> We can add methods to make this object behave like a dictionary by redefining d.keys() in terms of d.__dict__.keys() etc.
> 
> STEP 6)
> we can re-factor it a bit so that actually class Dummy is derived from dict.
> 
> Is this the part that people do not like?
> 
> 
> I would be happy if Python provided an efficient way to do something like STEP 4 without the problem I mentioned.
> Perhaps there is one and I ignore it. I do not necessarily require STEP5 and STEP6.
> 
> Use case:
> 
> settings = Dummy()
> settings.parameter1 = 'a'
> settings.parameter2 = 'b'
> etc.
> 
> if not settings.parameter1: do something ...
> 
> Massimo
> 
> 
> 
> On Jan 30, 2012, at 11:28 AM, Eric Snow wrote:
> 
>> On Mon, Jan 30, 2012 at 10:14 AM, Arnaud Delobelle <arnodel at gmail.com> wrote:
>>> 
>>> On Jan 30, 2012 4:23 PM, "Massimo Di Pierro" <massimo.dipierro at gmail.com>
>>> wrote:
>>>> 
>>>> I do not think the issue is whether the people who use that semantic
>>>> understand it or not. I can assure you they do and they know when it is
>>>> appropriate to use it or not. The issue is whether there is any value is
>>>> making it faster by including it in python or not. Because of the increasing
>>>> popularity of JS I think new users are starting to expect something like it
>>>> out of the box.
>>> 
>>> But this design decision in JavaScript is at the heart of many problems
>>> (e.g. simply looping over keys is a pain).  That it is widely used doesn't
>>> make it desirable. My experience with JavaScript is that we should keep this
>>> 'feature' out of Python. If people want it they can implement it very easily
>>> but encouraging them would be wrong.
>> 
>> +1
>
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