Josh Rosenberg <shadowranger+pyt...@gmail.com> added the comment:
I believe Tim Peters's concern with:
Set1 = Set1 & Set2
is misplaced, because he's drawn an analogy with the string concatenation
optimization, which *does* handle:
Str1 = Str1 + Str2
because the optimization is in the ceval loop and can check the subsequent
instruction for a "simple store", clearing the target's reference if the target
and the left operand are the same thing. This change doesn't try to handle this
case, so the only time the optimization takes place is when the only reference
to the left hand operand is on the stack (not in any named variable at all).
Tim's concern *would* make it unsafe to extend this patch to apply the target
clearing optimization, because at global scope, clearing the target would be
observable from __hash__/__eq__ (and from other threads); str is safe because
it holds the GIL the whole time, and can't invoke arbitrary Python level code
that might release it, set doesn't have that guarantee.
----------
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue36229>
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