Equivalent(x, y) sometimes prints as Equivalent(x, y) and sometimes as Equivalent(y, x). I haven't checked if it's not sorting the .args or if the issue is just with the printer.

Looks like a printer issue: the string printing of Equivalent falls back to the _print_Basic method, while other BooleanFunction subclasses such as And, Or have their own _print_And and _print_Or methods that sort the args.

Doesn't that mean it's an args ordering issue then?

I assume it's a design decision to have BooleanFunction not sort their args.

OK, looking at the code, it has nothing to do with BooleanFunction. BooleanFunction is just a Boolean Function, so it does sort its args (as it should). And and Or are lattice operations, but LatticeOp also sorts the args (it internally uses _argset in case .args is never used). It's Equivalent.new directly that does this, when it calls *set(args). It should use the same methodology as LatticeOp with _argset.

See https://github.com/sympy/sympy/pull/2560.

So can we remove the sorting from the _print_And and _print_Or methods?

Printing sorting and arg sorting aren't always the same. So we'd have to check.

We have moved issues to GitHub https://github.com/sympy/sympy/issues.

Migrated to http://github.com/sympy/sympy/issues/7180