def calculate(operator, num1, num2):
    """
    计算模块：执行数学计算
    返回: (计算结果, 是否错误)
    """
    if operator == '+':
        return num1 + num2, False
    elif operator == '-':
        return num1 - num2, False
    elif operator == '*':
        return num1 * num2, False
    elif operator == '/':
        if num2 == 0:
            return None, True
        return num1 / num2, False
    else:
        return None, True