Le samedi 30 avril 2011 à 13:36 -0400, Haz a écrit :
> So a few things to respond to...
>
>
> Tom:
> >Can you describe what went wrong with these branches? In particular,
> >why can the following naive strategy not work:
> >
> >1. Make the new system at least as good as the old one. That is
> >whenever I can write foo.is_bar I can write ask(foo, Q.bar) and I
> will
> >get at least as good an answer. In particular Symbol('x',
> >positive=True) should register automatically in the global
> assumptions
> >that x > 0 (if I understand correctly how the new system works...).
> >2. Replace all queries foo.is_bar by ask(foo, Q.bar).
> >3. Remove all implementations of the is_bar properties.
> >4. Remove all remaining remnants of the old system.
>
>
> This is what was largely tried with a branch last summer:
> - https://github.com/haz/sympy/tree/disconnect-assumptions-2
>
>
> There was push-back from the community since removing the
> Symbol('x', positive=True) syntax was largely frowned upon.
As I recall, the problems were that it was slow and broke a lot of
things. Symbol(x, positive=True) is easy to hack back in.
>
> Vinzent:
> >I think the only problem could be that ask() is much slower for
> >trivial queries, needs to be verified. This should be not too hard to
> >fix though.
>
>
> I don't think this is the case. Trivial queries (those that already
> have obvious implications) are compiled into quick reference
> dictionaries. In particular:
> - https://github.com/sympy/sympy/blob/master/sympy/assumptions/ask.py#L114
>
I think that the problem is with non-trivial queries, for which that
code does nothing.
>
> In the end, everything in the new assumption system is done with a
> particular assumption context -- design decisions must be made on
> which context is used in the ambiguous cases. There is also an issue
> (feature request) that makes things more natural that will be worked
> on after the assumptions shift has taken place -- things like the
> following would be possible:
>
>
> with <some assumptions on variables>:
> <do stuff>
>
>
> This gives the assumptions an explicit local scope. The branch from
> last summer was mostly filled with fixing test cases so they used the
> new assumption system (with the global assumptions), and then cleaned
> up after themselves.
>
>
> It should also be emphasized that it isn't just a simple notion of
> cleaning up assumptions after yourself -- the cache interacts in quite
> an intertwined way. Under the right assumptions, expressions will be
> simplified, and variables will be assigned certain values. If that's
> stored in the cache, changing any related assumption will invalidate
> the result. For this reason, I had to manually clear the cache on a
> regular basis. I think that the cache is only really needed for
> lengthy solve algorithms (so you don't recompute an expression over
> and over), in which case we can probably wipe it after any sympy
> command is invoked (i.e., only use the cache while sympy is computing
> things, and then flush it just before returning the result). However,
> the branches from last summer never made it this far.
>
The cache is the source of a lot of problems. We should try to use it as
little as possible, even if it's not really possible to remove it at
this time.
>
> I think a few things are generally universally accepted (correct me
> if I'm wrong though):
> 1. sympy/core/assumptions.py should be removed, and everything patched
> up to use sympy/assumptions/*
> 2. sympy/core/logic.py should be removed and replaced with
> sympy/logic/*
> 3. Assumptions should probably have the right references to be
> properly deleted from a context when their symbol is deleted -- this
> goes for any AssumptionContext (not just the global one)
>
I'm not sure about 1: sympy/core/assumptions.py and sympy/assumptions/*
should be merged, but I don't know how exactly the merge should happen.
>
> Ronan: Can you elaborate on this? (my memory is failing me at the
> moment)
> > * Remove the cumbersome and confusing Assume() syntax
The idea is to write '~Q.real(x)' instead of 'Assume(x, Q.real, False)'.
It's shorter, easier to understand and doesn't suggest wrongly that
creating the object does anything beyond that.
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