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

[Chris Kaynor]

On Mon, Jan 30, 2012 at 10:49 AM, Massimo Di Pierro <
massimo.dipierro at gmail.com> wrote:

> 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
>

getattr calls hasattr, which calls getattr again. You are hitting the
recursion limit before failing the test, due to hasattr failing with a
recursion error, thus returning false. You could refactor this to use
exceptions, such as:

def __getattr__(self, key):
    try:
        return object.__getattr__(self, key)
    except Exception:
        return 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?
>

The general discontent with the idea is step 4. Doing so leads to
abnormalities, such as d['clear'] != d.clear. The only ways to resolve such
abnormalities are to make using any keys/attributes which conflict illegal,
or to use different access methods for the two forms.


>
>
> 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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20120130/2ec49f14/attachment.html>