On 12/04/2021 06:53, 'Bruce Allen' via sympy wrote:
Hi David,
Thanks for picking this up. I wanted to comment that in your example,
the two symbols are defined differently:
Symbol("x", positive=True)
Symbol("x")
that is to say, with different assumptions. For the issue that I was
reporting, the two symbols that were defined with identical
assumptions, but were nevertheless being treated differently.
Well yes, my example was contrived, as I said. It mimicked what you
encountered because of the pickling fault.
However, my feeling is that some proportion of SymPy users will work
interactively - in one scope - without defining any Python functions. So
they might calculate a polynomial without regard to any assumptions, and
then wish to apply an assumption for one specific calculation and hit
the problem you encountered.
I do agree that if it's possible for a user to declare two different
symbols that have the same name (as the first argument of Symbol) but
which are not the same, this is likely to lead to confusion. On the
other hand, there must be a notion of the scope of a declaration.
I think all you need to do is redefine x using inconsistent assumptions.
This can be done in the same scope - there is no analogy with C or
Fortran, where you can't declare a variable twice in the same scope.
Indeed, you can call Symbol as many times as you like - it doesn't
define anything, it just sets up an instance of a class.
I would have thought that it would not be hard for the SymPy developers
to provide a switch that could fault the situation where a symbol is
redefined with the same name but different assumptions. I fully accept
that this would be mostly useful for interactive work.
I feel that gotchas of this sort could cause some real disaster because
it might be burried in a mass of algebra, and not be noticed before
papers or theses had been written.
David
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