Comment #4 on issue 1953 by asmeurer: inconsistency between "in" and "has"
http://code.google.com/p/sympy/issues/detail?id=1953
I don't think diff thing is necessarily a bug. a is b could return True
even when you wouldn't expect it because
of optimizations, as I said above. In this case, it has something to do
with creating a Derivative from the
method of the object as opposed to creating a new one from Derivative.
What I am saying is, don't change the == to is. That is wrong. It is not
expected that a is b should be True
when a == b is. It is only the reverse implication that should always hold.
By the way, I only disagree with the fix. I do think that a.has(b) should
probably be the same as b in a, unless
there is a good reason why it shouldn't be (speed maybe).
And anyway, I'm hoping that all this erroneous comparison of symbols with
different assumptions dicts will go
away with the new assumptions. But you'll have to talk to Vinzent and
Ronan and Christian, because I don't
really know what the plans are for that.
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