Make parameters fully pickleable

[pckroon]

It should be noted that the __getstate__ from the sympy parent classes is wrong, so I made a new one. Symbol.name survives pickling via the getnewargs method. We can't use that mechanism, since we need them to be passed to init.

[tBuLi]

I'm a bit surprised about the sympy getstate not being sufficient, because I think sympy objects are pickelable right? Do you have an explanation for this?

[pckroon]

If you have __slots__ you must also have a __gestate__ (and a __setstate__, bu there the Sympy's works: it just updates all the attributes after creation). Symbol.__getstate__ looks like this:

    def __getstate__(self):
        return {'_assumptions': self._assumptions}

Which of course is fine, if and only if you only have _assumptions. The rest of it's magice (name) has to be provided on object creation (not __init__, but __new__) which means this mechanism won't work. Instead, they're dealt with by __getnewargs__.

[tBuLi]

Thanks for the explanation.

And are you sure that the class attributes are basically broken by slots, and this global dict is the best workaround? If so, I will merge this.

[tBuLi]

whoops wrong button, premature close

[pckroon]

As it was it breaks on me saying that attribute _argument_index is read-only. This is I think the only workaround I saw. Maybe an alternative would be something like making a meta-class, or a singleton object that keeps track of this dict. Either way, overly complicated.

The only thing that's still troublesome is making more unnamed parameters after unpickling a bunch. But I'm not sure how big an issue that is.

[tBuLi]

That is indeed a bit troubling, but this is a deprecated feature anyway so I'm not sure how much time should be spend on it.

So I don't think pickling issues with automatically generated names are a big problem, since they are now against best practices anyway, so it's not unreasonable to demand people to be follow best practices when using multiprocessing for example. But would it be possible to modify __setstate__ to set argument_indices[cls] = max(argument_indices[cls], current_index)?

My biggest problem is that I want to avoid globals if in any way possible, so I would prefer any other solution over this but if there is no other way then we will do this since this feature is deprecated anyway.

[pckroon]

I'm not a fan of modifying setstate as you describe. It would require far too many braincycles to figure out how that namespace is going to work. I say we fix it when it breaks.

[tBuLi]

I agree with that.

But I just looked at the source of sympy.Dummy objects and they don't seem to have a problem with the class attribute, but it has to be a different one. See http://docs.sympy.org/1.0/_modules/sympy/core/symbol.html#Dummy.

Mimicking this is definitely the preferred solution.

[pckroon]

But that does mean your object will have a __dict__, negating some (all?) of the benefits involving __slots__

[tBuLi]

I finally had some time to check but Dummy's don't have __dict__'s, only __slots__. So I would prefer mimicking their solution to using a global variable.