construct SQL string from expression node

def expr_to_str(n, l=None):
    """
    construct SQL string from expression node
    """
    op = n[0]
    if op.startswith('_') and op.endswith('_'):
        op = op.strip('_')
        if op == 'var':
            return n[1]
        elif op == 'literal':
            if isinstance(n[1], basestring):
                return "'%s'" % n[1]
            return str(n[1])
        elif op == 'cast':
            return "(%s)::%s" % (expr_to_str(n[1]), n[2])
    elif op in '+-*/':
        return "(%s) %s (%s)" % (expr_to_str(n[1]), op, expr_to_str(n[2]))
    elif op == "extract":
        return "extract( %s from %s )" % (n[1], expr_to_str(n[2]))
    else:
        arg = ','.join(map(expr_to_str, n[1:]))
        return "%s(%s)" % (op, arg)