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Date: Fri, 24 Jun 2011 16:08:35 -0500
From: Tycho Andersen <tycho@tycho.ws>
To: Ned Deily <nad@acm.org>
Subject: Re: Interpreting Left to right?
References: <BANLkTimcPAjQP4JJk=OAbrkLFfd2AtndwQ@mail.gmail.com> <iu1aaq$rni$1@dough.gmane.org> <4E043953.5080705@stoneleaf.us> <20110624195300.GJ6075@point.cs.wisc.edu> <4E04EFD4.6050309@stoneleaf.us> <20110624200618.GK6075@point.cs.wisc.edu> <nad-C01F34.13242424062011@news.gmane.org>
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On Fri, Jun 24, 2011 at 01:24:24PM -0700, Ned Deily wrote:
> In article <20110624200618.GK6075@point.cs.wisc.edu>,
>  Tycho Andersen <tycho@tycho.ws> wrote:
> > Yes, I understand that, but I guess I don't understand *why* things
> > are done that way. What is the evaluation order principle at work
> > here? I would have expected:
> > 
> > tmp = {}
> > x['huh'] = tmp # NameEror!
> > 
> > That is, the right hand sides of assignments are evaluated before the
> > left hand sides. That is (somehow?) not the case here.
> 
> http://docs.python.org/py3k/reference/simple_stmts.html#assignment-statements

Perhaps I'm thick, but (the first thing I did was read the docs and) I
still don't get it. From the docs:

"An assignment statement evaluates the expression list (remember that
this can be a single expression or a comma-separated list, the latter
yielding a tuple) and assigns the single resulting object to each of
the target lists, from left to right."

For a single target, it evaluates the RHS and assigns the result to
the LHS. Thus

x = x['foo'] = {}

first evaluates

x['foo'] = {}

which should raise a NameError, since x doesn't exist yet. Where am I
going wrong?

Thanks,

\t