Comment 1 for bug 242813

I think I am wrong about the API being a limiting factor here: with smarter code, we could pretty easily avoid the recursion problems by changing Store._set_expr() to something like this:

    ...
    left_select = self._get_select()
    right_select = other._get_select()
    # Try to flatten nested set expressions.
    if (isinstance(left_select, expr_cls) and left_select.all == all and
        left_select.limit is Undef and left_select.offset is Undef):
        subexprs = left_select.exprs + (right_select,)
    else:
        subexprs = (left_select, right_select)
    expr = expr_cls(*subexprs, all=all)
    ...

The idea being to take advantage of left associativity of set operations to generate a shallower expression tree. It looks like this would bring the same benefit to the SQLObject compatibility layer with no changes.