Re: Is vars() the most useless Python built-in ever?

Newsgroups	comp.lang.python
Date	2015-11-30 19:45 -0800
References	<565cf141$0$1612$c3e8da3$5496439d@news.astraweb.com>
Message-ID	<98d1c37d-0c25-45fd-a06f-244eca5e3f6a@googlegroups.com> (permalink)
Subject	Re: Is vars() the most useless Python built-in ever?
From	Josef Pktd <josef.pktd@gmail.com>

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On Monday, November 30, 2015 at 8:01:14 PM UTC-5, Steven D'Aprano wrote:
> I'm trying to understand why vars() exists. Does anyone use it?
> 
> Every time I try to use it, I find it doesn't quite do what I want. And even
> if it did, there are more obvious and/or correct alternatives.
> 
> For instance, I want to check whether a particular name is an instance
> attribute. So first I write:
> 
> "name" in obj.__dict__
> 
> but when I see the dunder name I think that's an implementation detail. And
> sure enough, not all instances have a __dict__ (e.g. if it uses __slots__
> instead) and so I re-write it as:
> 
> "name" in vars(obj)
> 
> but that also fails if obj has no instance __dict__.
> 
> But why am I looking just in the instance __dict__? Chances are I should be
> looking for the attribute *anywhere* in the instance/class/superclass
> hierarchy: 
> 
> hasattr(obj, "name")
> 
> Or, if you are worried about triggering dynamic attributes using
> __getattr__, you can do this:
> 
> sentinel = object()
> inspect.getattr_static(obj, "name", sentinel) is not sentinel
> 
> which only checks for pre-existing attributes without triggering
> __getattr__, __getattribute__, or the descriptor protocol.
> 
> 
> Either way, vars() doesn't solve the problem. What problem does it solve?

I'm using dir and vars pretty often when I'm trying to figure out new code or review a pull request, or when I'm reviewing code that I don't remember.

Both are convenient for quickly checking which attributes are available at the moment. I never realized that there might be issues with "fancy" problems. 

`vars` is convenient, for example, to check which attributes have been initialized to None, and which already have assigned values. And checking again after some calculations, I roughly see which attributes have been added or changed in the mean time.

aside: I'm boring and like code that has no magic, especially if I have to maintain it.

Josef

> 
> 
> 
> -- 
> Steven

Thread

Is vars() the most useless Python built-in ever? Steven D'Aprano <steve@pearwood.info> - 2015-12-01 12:00 +1100
  Re: Is vars() the most useless Python built-in ever? Josef Pktd <josef.pktd@gmail.com> - 2015-11-30 19:45 -0800
  Re: Is vars() the most useless Python built-in ever? Rick Johnson <rantingrickjohnson@gmail.com> - 2015-11-30 21:54 -0800
    Re: Is vars() the most useless Python built-in ever? Marko Rauhamaa <marko@pacujo.net> - 2015-12-01 09:34 +0200
      Re: Is vars() the most useless Python built-in ever? Rick Johnson <rantingrickjohnson@gmail.com> - 2015-12-11 13:06 -0800
    Re: Is vars() the most useless Python built-in ever? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-12-01 18:55 +1100
      Re: Is vars() the most useless Python built-in ever? Rick Johnson <rantingrickjohnson@gmail.com> - 2015-12-01 12:33 -0800
        Re: Is vars() the most useless Python built-in ever? Steven D'Aprano <steve@pearwood.info> - 2015-12-02 11:57 +1100
        Re: Is vars() the most useless Python built-in ever? Ian Kelly <ian.g.kelly@gmail.com> - 2015-12-01 22:48 -0600
          Re: Is vars() the most useless Python built-in ever? Rick Johnson <rantingrickjohnson@gmail.com> - 2015-12-11 14:13 -0800
            Re: Is vars() the most useless Python built-in ever? Steven D'Aprano <steve@pearwood.info> - 2015-12-12 15:44 +1100
              Re: Is vars() the most useless Python built-in ever? Chris Angelico <rosuav@gmail.com> - 2015-12-12 16:56 +1100
              Re: Is vars() the most useless Python built-in ever? Rick Johnson <rantingrickjohnson@gmail.com> - 2015-12-14 18:33 -0800
      Re: Is vars() the most useless Python built-in ever? Ben Finney <ben+python@benfinney.id.au> - 2015-12-02 12:47 +1100
    Re: Is vars() the most useless Python built-in ever? John Gordon <gordon@panix.com> - 2015-12-01 16:56 +0000
      Re: Is vars() the most useless Python built-in ever? Rick Johnson <rantingrickjohnson@gmail.com> - 2015-12-01 13:15 -0800
        Re: Is vars() the most useless Python built-in ever? "Albert Visser" <albert.visser@gmail.com> - 2015-12-02 19:08 +0100
    Re: Is vars() the most useless Python built-in ever? Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-12-02 13:14 +1300
  Re: Is vars() the most useless Python built-in ever? eryk sun <eryksun@gmail.com> - 2015-12-01 01:22 -0600
    Re: Is vars() the most useless Python built-in ever? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-12-01 18:57 +1100
  Re: Is vars() the most useless Python built-in ever? Manolo Martínez <manolo@austrohungaro.com> - 2015-12-01 09:44 +0100
    Re: Is vars() the most useless Python built-in ever? Steven D'Aprano <steve@pearwood.info> - 2015-12-02 12:12 +1100
      Re: Is vars() the most useless Python built-in ever? Peter Otten <__peter__@web.de> - 2015-12-02 08:51 +0100
  Re: Is vars() the most useless Python built-in ever? Peter Otten <__peter__@web.de> - 2015-12-01 11:40 +0100
  Re: Is vars() the most useless Python built-in ever? Manolo Martínez <manolo@austrohungaro.com> - 2015-12-01 22:31 +0100
  Re: Is vars() the most useless Python built-in ever? Peter Otten <__peter__@web.de> - 2015-12-01 23:41 +0100
  Re: Is vars() the most useless Python built-in ever? boB Stepp <robertvstepp@gmail.com> - 2015-12-01 20:20 -0600
  Re: Is vars() the most useless Python built-in ever? wxjmfauth@gmail.com - 2015-12-01 23:46 -0800
  Re: Is vars() the most useless Python built-in ever? Serhiy Storchaka <storchaka@gmail.com> - 2015-12-02 10:22 +0200
  Re: Is vars() the most useless Python built-in ever? Manolo Martínez <manolo@austrohungaro.com> - 2015-12-02 10:09 +0100
  Re: Is vars() the most useless Python built-in ever? Chris Angelico <rosuav@gmail.com> - 2015-12-02 20:28 +1100
  Re: Is vars() the most useless Python built-in ever? Chris Angelico <rosuav@gmail.com> - 2015-12-02 20:33 +1100
  Re: Is vars() the most useless Python built-in ever? Manolo Martínez <manolo@austrohungaro.com> - 2015-12-02 10:40 +0100
  Re: Is vars() the most useless Python built-in ever? Peter Otten <__peter__@web.de> - 2015-12-02 11:48 +0100
  Re: Is vars() the most useless Python built-in ever? Chris Angelico <rosuav@gmail.com> - 2015-12-02 22:02 +1100
  Re: Is vars() the most useless Python built-in ever? eryk sun <eryksun@gmail.com> - 2015-12-02 05:34 -0600
  Re: Is vars() the most useless Python built-in ever? Serhiy Storchaka <storchaka@gmail.com> - 2015-12-04 10:42 +0200

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