python写一个系统根据题目难度，为每个题目赋分，总分为100。系统随机生成四则运算表达式，让用户计算。系统自动对计算结果进行判断对错，系统自动计算成绩并返回用户测试所用时长和最终得分

好的，我可以为您提供一个简单的代码来实现这个系统。代码如下：

import random
import time

# 随机生成四则运算表达式
def generate_expression(difficulty):
    if difficulty == "easy":
        num1 = random.randint(1, 20)
        num2 = random.randint(1, 20)
        operator = random.choice(["+", "-"])
    elif difficulty == "medium":
        num1 = random.randint(10, 50)
        num2 = random.randint(1, 10)
        operator = random.choice(["+", "-", "*"])
    else:
        num1 = random.randint(50, 100)
        num2 = random.randint(10, 20)
        operator = random.choice(["+", "-", "*", "/"])
    
    expression = f"{num1} {operator} {num2}"
    return expression

# 计算表达式的结果
def calculate_result(expression):
    try:
        result = eval(expression)
        return result
    except:
        return None

# 判断用户输入的答案是否正确
def check_answer(user_answer, correct_answer):
    if user_answer == correct_answer:
        return True
    else:
        return False

# 为每个题目赋分
def assign_score(difficulty):
    if difficulty == "easy":
        score = 20
    elif difficulty == "medium":
        score = 30
    else:
        score = 50
    return score

# 主函数
def main():
    difficulty = input("请选择难度（easy/medium/hard）：")
    score = 0
    total_score = 100
    start_time = time.time()
    for i in range(5):
        expression = generate_expression(difficulty)
        print(f"第{i+1}题：{expression} = ", end="")
        user_answer = input()
        correct_answer = calculate_result(expression)
        if check_answer(user_answer, correct_answer):
            print("回答正确！")
            score += assign_score(difficulty)
        else:
            print("回答错误！")
    
    end_time = time.time()
    time_used = round(end_time - start_time, 2)
    final_score = round(score / 5, 2)
    print(f"测试用时：{time_used}秒，最终得分：{final_score}/{total_score}")

if __name__ == "__main__":
    main()

这个系统会要求用户选择难度，根据难度随机生成四则运算表达式，并让用户计算结果。然后系统会自动根据用户的答案进行判断，计算得分并返回测试用时和最终得分。