编写Arithmetic Arranger函数触发TypeError错误求助

需求背景 #

需要实现一个返回算术表达式排列格式的函数arithmetic_arranger：

• 输入arithmetic_arranger(["32 + 698", "3801 - 2", "45 + 43", "123 + 49"])时，输出对齐的表达式格式
• 当第二个参数为True时，额外显示计算结果

用户代码 #

def arithmetic_arranger(problems, show = True):
  #["1 + 3", "1 - a", "1 * 4"]
  arranged_problems = []
  #too many problems supplied
  if len(problems)>5:
    return "Error: Too many problems."
  for i, ele in enumerate(problems):
    sub = ele.split()
    num1 = sub[0]
    oper = sub[1]
    num2 = sub[2]
    #appropriate operators "+","-"
    if sub[1] not  in "-+":
      return "Error: Operator must be '+' or '-'."
    #only contain digits
    if sub[0].isdigit() and sub[2].isdigit() is False:
        return "Error: Numbers must only contain digits."
    #max 4 digits
    if len(sub[0])>4 or len(sub[2])>4:
      return "Error: Numbers cannot be more than four digits."
      
    ray = "-"

    maxlen_line = max(len(num1), len(num2))
    ach = maxlen_line + 2

    line_num1 = f"{num1:>{ach}}"
    line_num2 = oper + f"{num2:>{ach-1}}"
    line_rays = ray*ach
    
    try:
      arranged_problems[0] += (" "*4) + line_num1
    except IndexError:
      arranged_problems.append(line_num1)
    try:
      arranged_problems[1] += (" "*4) + line_rays
    except IndexError:
      arranged_problems.append(line_rays)
    try:
      arranged_problems[2] += (" "*4) + line_num2
    except IndexError:
      arranged_problems.append(line_num2)
    answ = 0 
    if oper == "+":
      answ = int(num1) + int(num2)
    else:
      answ = int(num1) - int(num2)
    
    line_answ = f"{str(answ):>{ach}}"
    try:
      arranged_problems[3] += (" "*4) + line_answ
    except IndexError:
      arranged_problems.append(line_answ)
    if show == True:
      arranged_problems = f"{arranged_problems[0]}\n{arranged_problems[1]}\n{arranged_problems[2]}\n{arranged_problems[3]}"
    else:
      arranged_problems = f"{arranged_problems[0]}\n{arranged_problems[1]}\n{arranged_problems[2]}"
      
  
  return arranged_problems

触发错误 #

Traceback (most recent call last):
  File "/home/runner/boilerplate-arithmetic-formatter/main.py", line 7, in <module>
    print(arithmetic_arranger(["32 + 698", "3801 - 2", "45 + 43", "123 + 49"]))
  File "/home/runner/boilerplate-arithmetic-formatter/arithmetic_arranger.py", line 32, in arithmetic_arranger
    arranged_problems[0] += (" "*4) + line_num1
TypeError: 'str' object does not support item assignment

错误原因 #

核心问题是变量类型在循环中被意外修改：

• 初始时arranged_problems是列表，用来存储每一行的内容（数字行、分隔线行、运算符行、结果行）
• 第一次循环处理完第一个表达式后，执行了arranged_problems = f"{...}"，将列表直接转换成了字符串
• 第二次循环开始时，arranged_problems已经是字符串类型，此时尝试arranged_problems[0] += ...，本质是试图修改字符串的索引位置内容，但Python中字符串是不可变类型，不支持这种操作，因此触发TypeError

修复方案 #

将字符串拼接的逻辑移到循环结束之后，循环过程中始终用列表维护各行内容：

1. 循环内只负责将每个表达式的对应行追加到独立的行列表中
2. 所有表达式处理完成后，再根据show参数决定是否包含结果行
3. 最后将各行列表用空格连接，再组合成带换行符的最终字符串返回

修正后的代码 #

def arithmetic_arranger(problems, show=False):
    # 检查问题数量是否超过限制
    if len(problems) > 5:
        return "Error: Too many problems."
    
    # 初始化四行的存储列表
    line1 = []
    line2 = []
    line3 = []
    line4 = []
    
    for ele in problems:
        sub = ele.split()
        num1 = sub[0]
        oper = sub[1]
        num2 = sub[2]
        
        # 检查运算符是否合法
        if oper not in "-+":
            return "Error: Operator must be '+' or '-'."
        # 检查数字是否仅包含数字字符
        if not (num1.isdigit() and num2.isdigit()):
            return "Error: Numbers must only contain digits."
        # 检查数字长度是否超过4位
        if len(num1) > 4 or len(num2) > 4:
            return "Error: Numbers cannot be more than four digits."
        
        maxlen_line = max(len(num1), len(num2))
        ach = maxlen_line + 2
        
        # 格式化每一行的内容
        line1.append(f"{num1:>{ach}}")
        line2.append(f"{oper}{num2:>{ach-1}}")
        line3.append("-" * ach)
        
        # 计算结果（如果需要显示）
        if show:
            answ = int(num1) + int(num2) if oper == "+" else int(num1) - int(num2)
            line4.append(f"{str(answ):>{ach}}")
    
    # 组合所有行
    arranged_problems = [
        "    ".join(line1),
        "    ".join(line2),
        "    ".join(line3)
    ]
    if show:
        arranged_problems.append("    ".join(line4))
    
    return "\n".join(arranged_problems)

内容的提问来源于stack exchange，提问作者Pablovick