Piecewise fails with unevaluated relational

[oscarbenjamin]

In [17]: Piecewise((1, Lt(1, 2, evaluate=False)), (2, True))
---------------------------------------------------------------------------
TypeError: Invalid comparison of non-real zoo

This is dues to the _canonical_coeff function somehow making incorrect presumptions about the relational:

ipdb> ll
     46 def _canonical_coeff(rel):
     47     # return -2*x + 1 < 0 as x > 1/2
     48     # XXX make this part of Relational.canonical?
     49     rel = rel.canonical
     50     if not rel.is_Relational or rel.rhs.is_Boolean:
     51         return rel  # Eq(x, True)
     52     b, l = rel.lhs.as_coeff_Add(rational=True)
     53     m, lhs = l.as_coeff_Mul(rational=True)
     54     rhs = (rel.rhs - b)/m
     55     if m < 0:
     56         return rel.reversed.func(lhs, rhs)
---> 57     return rel.func(lhs, rhs)
     58 

ipdb> p m
0
ipdb> p rhs
zoo

[1e9abhi1e10]

Can we change the definition of _canonical_coeff with respect to relational ?
But when we write the1<2 instead of Lt(1, 2, evaluate=False) it doesn't give any error!

In [3] : Piecewise((1,1<2), (2, True))
Out[4] : 1

How should I move forward for opening the PR for that?

[oscarbenjamin]

How should I move forward for opening the PR for that?

It isn't possible to write a PR that changes what code the user chooses to execute.

[smichr]

line 53 above could be changed to m, lhs = l.as_coeff_Mul(rational=True) if l else S.One, l. I wish S.Zero.as_coeff_Mul() gave (1,0) instead of (0, 1)...but that is a way to work around it.