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Re: using __getitem()__ correctly

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
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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].

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Thread

using __getitem()__ correctly "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2015-12-30 12:57 +0000
  Re: using __getitem()__ correctly Chris Angelico <rosuav@gmail.com> - 2015-12-31 00:11 +1100
    Re: using __getitem()__ correctly "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2015-12-30 14:40 +0000
      Re: using __getitem()__ correctly Ian Kelly <ian.g.kelly@gmail.com> - 2015-12-30 08:35 -0700
        Re: using __getitem()__ correctly "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2015-12-30 16:58 +0000
          Re: using __getitem()__ correctly Ian Kelly <ian.g.kelly@gmail.com> - 2015-12-30 13:40 -0700
            Re: using __getitem()__ correctly "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2015-12-30 22:54 +0000
              Re: using __getitem()__ correctly "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2015-12-30 22:58 +0000
              Re: using __getitem()__ correctly Ben Finney <ben+python@benfinney.id.au> - 2015-12-31 10:13 +1100
                Re: using __getitem()__ correctly "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2015-12-30 23:18 +0000
                Re: using __getitem()__ correctly Steven D'Aprano <steve@pearwood.info> - 2015-12-31 10:50 +1100
                Re: using __getitem()__ correctly Ben Finney <ben+python@benfinney.id.au> - 2015-12-31 11:21 +1100
                Re: using __getitem()__ correctly "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2015-12-31 11:30 +0000
                Re: using __getitem()__ correctly Ben Finney <ben+python@benfinney.id.au> - 2015-12-31 22:51 +1100
                Re: using __getitem()__ correctly Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2015-12-31 12:12 +0000
                Re: using __getitem()__ correctly "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2015-12-31 13:39 +0000
                Re: using __getitem()__ correctly Steven D'Aprano <steve@pearwood.info> - 2016-01-01 02:03 +1100
                Re: using __getitem()__ correctly "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2015-12-31 11:17 +0000
                Re: using __getitem()__ correctly Steven D'Aprano <steve@pearwood.info> - 2016-01-01 01:43 +1100
              Re: using __getitem()__ correctly Ian Kelly <ian.g.kelly@gmail.com> - 2015-12-30 17:31 -0700
                Re: using __getitem()__ correctly "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2015-12-31 12:45 +0000

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