Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Ian Kelly <ian.g.kelly@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: using __getitem()__ correctly
Date: Wed, 30 Dec 2015 08:35:57 -0700
Lines: 47
Message-ID: <mailman.76.1451489799.11925.python-list@python.org>
References: <n60kcc$h89$1@dont-email.me> <mailman.70.1451481087.11925.python-list@python.org> <n60qcp$rtp$2@dont-email.me>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8
In-Reply-To: <n60qcp$rtp$2@dont-email.me>
Precedence: list
Xref: csiph.com comp.lang.python:101001

On Dec 30, 2015 7:46 AM, "Charles T. Smith" <cts.private.yahoo@gmail.com> wrote:
> As is so often the case, in composing my answer to your question, I discovered
> a number of problems in my class (e.g. I was calling __getitem__() myself!), but
> I'm puzzled now how to proceed.  I thought the way you avoid triggering __getattr__()
> from within that was to use self.__dict__[name] but that doesn't work:
>
>   (PDB)p self.attrs.keys()
>   ['mcc', 'abc']
>   (PDB)p self.attrs.__dict__['abc']
>   *** KeyError: KeyError('abc',)

What leads you to believe that this is triggering a call to
__getattr__? The KeyError probably just means that the key 'abc'
wasn't found in the dict.

> class attrdict(dict):
>     def __init__ (self, name = None):
>         if name:
>             self.update (name)
>         print "attrdict: instantiated: ", name
>
>     # AutoVivification
>     def __getattr__ (self, name):
>         print "attrdict:av:__getattr__: entered for ", name      #, " in ", self
>         #if not name in self.__dict__.keys():
>         if not name in self.keys():

Use the "not in" operator, e.g. "if name not in self.keys()".

>             print "attrdict:av:__getattr__: autovivifying ", name
>             #self.__dict__.__setitem__ (name, self.__class__())
>             #self.__setitem__ (name, self.__class__())
>             self.__setattr__ (name, self.__class__())

No reason to explicitly call __setitem__ or __setattr__ here. I'd
probably just do self[name] = self.__class__()

>         #return self.__getitem__(name)
>         #return self.__dict__.__getitem__(name)
>         return self.__getattribute__ (name)

You shouldn't call __getattribute__ from __getattr__, because
__getattr__ is called from __getattribute__, so this would cause an
infinite loop.

Based on the preceding, you probably want to return the value you just
set in the dict, correct? So just return self[name].