Antoine Pitrou added the comment:
> I should also point out that the marshal protocol takes care to be
> able to serialize lists, sets and frozensets correctly, the latter
> being added in version 2.4. This despite the fact that code objects
> don't make use of these.
Code objects do use frozensets:
>>> def f(x):
... return x in {1,2,3,4,5,6}
...
>>> dis.dis(f)
2 0 LOAD_FAST 0 (x)
3 LOAD_CONST 7 (frozenset({1, 2, 3, 4, 5, 6}))
6 COMPARE_OP 6 (in)
9 RETURN_VALUE
I don't think marshal supports any type that isn't (or hasn't been)
used in code objects.
> Obviously, this shows that marshal is still expected to work and be
> useful even if not for pickling code objects.
The module officially intended for general-purpose serialization is
pickle; if you use marshal for such a purpose, you're on your own.
If you think pickle is not good enough, your improvements are welcome.
> As expected, there is no difference between version 3 and 4 unless
> I employ the recode module to fold common subobjects. This brings
> an additional saving of some 3% bringing the total reduction up to
> 28% and 18% respectively.
3% doesn't sound like a worthwhile improvement at all.
> The trick of using a bit flag on the type to indicate a slot
> reservation in the instance list is one that has been in use in
> CCP´s own "Marshal" format, a proprietary serialization format
> based on marshal back in 2002 (adding many more special opcodes
> and other stuff)
Why don't you release your "proprietary marshal" on pypi? You would
get feedback and a sense of whether people are interested in your
approach.
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<http://bugs.python.org/issue16475>
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