New submission from benrg <benrud...@gmail.com>:
>From 3.3 on, the expression () is compiled to BUILD_TUPLE 0 instead of
>LOAD_CONST. That's probably fine and I suppose it's slightly more efficient to
>avoid adding an entry to the constant table.
The problem is that BUILD_TUPLE 0 is not treated as a constant for folding
purposes, so any otherwise constant expression that contain () ends up
compiling into O(n) bytecode instructions instead of 1. I think this is a bug
(rather than an enhancement) because it seems unlikely to be the intended
behavior.
In 3.2 an earlier, and in 2.7, the constant-folding behavior is different, and
many constant tuples aren't recognized at compile time for reasons unclear to
me, but there are definitely cases it will fold that 3.3+ won't. For example,
"x in {(), None}" tests a frozenset in 3.2, but builds a set at run time in
3.3+.
----------
components: Interpreter Core
messages: 309739
nosy: benrg
priority: normal
severity: normal
status: open
title: Empty tuples are not optimized as constant expressions
type: performance
versions: Python 3.4, Python 3.5, Python 3.6, Python 3.7
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue32525>
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