In article <20110624210835.gl6...@point.cs.wisc.edu>, Tycho Andersen <ty...@tycho.ws> wrote:
> On Fri, Jun 24, 2011 at 01:24:24PM -0700, Ned Deily wrote: > > In article <20110624200618.gk6...@point.cs.wisc.edu>, > > Tycho Andersen <ty...@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, n...@acm.org -- http://mail.python.org/mailman/listinfo/python-list