Raymond Hettinger <rhettin...@users.sourceforge.net> added the comment:
Thanks for the patch. I had looked at this long ago when I first added the
ROT2 optimization and the ROT3/ROT2 optimization. It wasn't included because
it wasn't worth the added complexity in the peepholer logic and because there
were concerns about executing internally in a different order than specified by
the code.
Since LOAD_NAME and LOAD_GLOBAL are subject to user control, changing their
order of evaluation causes a visible change in semantics. For example, the
following result is different than before the patch.
>>> class Dict(dict):
... def __getitem__(self, key):
... print(key)
... return dict.__getitem__(self, key)
...
>>> ns = Dict()
>>> exec('c=1; d=2; a,b=c,d', globals(), ns)
d
c
For the most part, I'm not too excited about the patch because
* it is limited to very simple cases that already have some optimization
* it needs to be limited even further to be semantically neutral
* it adds complexity to a part of the peepholer that is already a bit too
complicated (the more peephole assumptions we make, the harder it is to
maintain, especially when opcodes are added, deleted, or changed).
* changing order of execution starts to venture into territory that we've
stayed away from (on purpose).
----------
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue10648>
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