times: elementwise multiplication broken on SymPy 1.9

[cbm755]

Consider

 syms x
 A = [1 x];
 B = [2 3];
 A .* B

On SymPy 1.9, I get:

Python exception: AssertionError
    occurred at line 6 of the Python code block:
    return x.multiply_elementwise(y)

Looks like an upstream issue but its fixed in the later SymPy 1.10.x. Should bisect and ensure upstream has a unit test for this...

If so (rather than our fault), maybe we want to implement a blocklist or at leat a warnlist on "known bad" SymPy versions. E.g., in sympref diagnose.

[chrisjohgorman]

A git bisect leaves [67b59a8f591180fb1e5ecd02e793b929541327e1] multiply_elementwise error fixed as the commit which fixes the issue. Looking at the commit, I don't see a unit test added. Of course it's possible to have added the unit test in a different commit. Where would I go to add the unit test?

diff --git a/sympy/matrices/repmatrix.py b/sympy/matrices/repmatrix.py
index 2f0cdcd6ca..84cde91b1e 100644
--- a/sympy/matrices/repmatrix.py
+++ b/sympy/matrices/repmatrix.py
@@ -239,8 +239,9 @@ def _eval_matrix_mul(self, other):
         return classof(self, other)._fromrep(self._rep * other._rep)
 
     def _eval_matrix_mul_elementwise(self, other):
-        rep = self._rep.mul_elementwise(other._rep)
-        return classof(self, other)._fromrep(rep)
+        selfrep, otherrep = self._rep.unify(other._rep)
+        newrep = selfrep.mul_elementwise(otherrep)
+        return classof(self, other)._fromrep(newrep)
 
     def _eval_scalar_mul(self, other):
         rep, other = self._unify_element_sympy(self._rep, other)

[chrisjohgorman]

I looked around at sympy and found that pull request from the author also included a commit to the sympy/matrices/tests/test_commonmatrix.py. It mentions the issue #22353. I decided to run this test case, by hand in sympy-1.9 where it fails. It now runs properly in 1.10.

[cbm755]

Anything we should do here for 3.0.0? We could have a known_bad list of SymPy versions, maintained in sympref and checked similar to how we check the minimum sympy version... Or just ignore it?

[alexvong243f]

Actually, for this particular issue, we can workaround it by using hadamard_product(x, y) instead of x.multiply_elementwise(y) for sympy 1.9 because the bug for hadamard_product sympy/sympy#8557 has been fixed since Mar 2020 (sympy 1.9 was released on Oct 2021).

[cbm755]

So could we keep using hadamard_product when SymPy >= 1.9? That sounds like a nice fix to me!

[alexvong243f]

Yes, hadamard_product works fine and passes all tests in sympy 1.9. Should we use hadamard_product instead of multiply_elementwise in sympy >1.9 as well?

[cbm755]

Sure, try it? If tests pass, it should be good. Semantically correct is good!