def _get_const_binary_expr(stencil_ir, func_ir, index_def):
    """evaluate constant binary expr if possible
    otherwise, raise GuardException
    """
    require(isinstance(index_def, ir.Expr) and index_def.op == 'binop')
    arg1 = _get_const_index_expr_inner(stencil_ir, func_ir, index_def.lhs)
    arg2 = _get_const_index_expr_inner(stencil_ir, func_ir, index_def.rhs)
    op = OPERATORS_TO_BUILTINS[index_def.fn]
    return eval("{}{}{}".format(arg1, op, arg2))
evaluate constant binary expr if possible
    otherwise, raise GuardException
    
numba/stencils/stencilparfor.py