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[Python-ideas] RFC: PEP: Add dict.__version__

Victor Stinner victor.stinner at gmail.com
Sat Jan 9 04:58:00 EST 2016 
2016-01-09 9:57 GMT+01:00 Serhiy Storchaka <storchaka at gmail.com>:
>>> This also can be used for better detecting dict mutating during
>>> iterating:
>>> https://bugs.python.org/issue19332.
> (...)
>
> This makes Raymond's objections even more strong.

Raymond has two major objections: memory footprint and performance. I
opened an issue with a patch implementing dict__version__ and I ran
pybench:
https://bugs.python.org/issue26058#msg257810

pybench doesn't seem reliable: microbenchmarks on dict seems faster
with the patch, it doesn't make sense. I expect worse or same
performance.

With my own timeit microbenchmarks, I don't see any slowdown with the
patch. For an unknown reason (it's really strange), dict operations
seem even faster with the patch.

For the memory footprint, it's clearly stated in the PEP that it adds
8 bytes per dict (4 bytes on 32-bit platforms). See the "dict subtype"
section which explains why I proposed to modify directly the dict
type.

IMHO adding 8 bytes per dict is worth it. See for example
microbenchmarks on func.specialize() which rely on dict.__version__ to
implement efficient guards on namespaces:
https://faster-cpython.readthedocs.org/pep_specialize.html#example

"1.6 times" (155 ns => 95 ns) is better than a few percent as fast
usually seen when optimizing dict operations.

Victor
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