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Re: Interpreting Left to right?

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  Re: Interpreting Left to right? Ned Deily <nad@acm.org> - 2011-06-24 15:10 -0700

#8417 — Re: Interpreting Left to right?

In article <20110624210835.GL6075@point.cs.wisc.edu>,
 Tycho Andersen <tycho@tycho.ws> wrote:

> 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-statement
> > s
> 
> 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?

"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."

Also, remember that in Python the "=" is not part of an expression.  
It's a token in the assignment statement.

-- 
 Ned Deily,
 nad@acm.org

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