订单系统中禁用指定内置函数时解决字符串转float的ValueError

开发一个管理员上传商品、顾客下单的订单系统，实现了place_order函数。在将列表中的字符串转换为float类型时，出现如下报错：

ValueError: could not convert string to float: '40.0,'

作业要求不能使用join()、map()、enumerate()、replace()这些内置函数，求去除数字后逗号的方法。

完整下单函数代码 #

def place_order():

try:
    fileHandler= open('groceries_list.txt','r')
    
except:
    print('File cannot be opened.')
    exit()
    
gro=fileHandler.readlines()

if len(gro) == 0:
    print("Sorry! There are no groceries in our store yet.")        
else:
    repeat = True

while repeat == True:
    try:
        fo= open('groceries_list.txt')
        fr= fo.readlines()
    except:
        print('File cannot be read.')
        
    order_list=[]
    total=0
    onp=[]
    view_gro()
    product= input('Please enter groceries name you want to purchase: ').upper()
   
    for line in fr:
        line=line.rstrip()
        if not product in line:
            continue
        print(line)
        line=line.split()
        print('The price for',product,'is RM',(line[3]))
        ######################need to convert line[3] in list to float
        line[3]=float(line[3])
        total=total+line[3]
        order_list.append(product)
        print('Product successful added to you shopping cart!')
        again= input('Do you need anything else? YES for continue or NO for quit.').upper()
        if again == 'YES':
            continue
        elif again == 'NO':
            print("Here's your shopping cart item")
            print(order_list)
            print('Total amount for your purchasing is RM',total)
            try:
                while True: 
                    pay=int(input('You pay: RM '))
                    balance= total-pay
                    if balance == 0:
                        print('Pay successfully! Thanks for your purchasing')
                        fw= open('order_list_c.txt','w')
                        fw.write(order_list)
                        fw.write('\n')
                        fw.close()
                        repeat=False
                        #break
                    else:
                        print('Pay unsuccessfully! Please try again.')
                        continue
            except ValueError:
                print('Please enter numeric proper amount.')
                continue
        else:
            print('Please enter YES or NO')
            print('==============================')
            continue
       # else:
        #    print('Product not in list.')
         #   continue
                
    #else:
     #   print('No products found. Try again.')
      #  continue
fo.close()        
fileHandler.close() 

出错代码片段 #

for line in fr:
        line=line.rstrip()
        if not product in line:
            continue
        print(line)
        line=line.split()
        print('The price for',product,'is RM',(line[3]))
        ######## need to convert line[3] in list to float but contains comma ######
        line[3]=float(line[3])
        total=total+line[3]

报错信息 #

line[3]=float(line[3])
       ValueError: could not convert 
       string to float: '40.0,'

数据示例 #

['FOOD', 'vege', 2022, 40.0, 'fresh']

解决方案 #

以下两种方法均不使用禁用的内置函数，可解决字符串转float的问题：

方法1：遍历字符构建纯净字符串 #

逐个检查价格字符串的每个字符，只保留非逗号的字符，再转换为float：

price_str = line[3]
clean_str = ''
for char in price_str:
    if char != ',':
        clean_str += char
line[3] = float(clean_str)

方法2：字符串切片（针对逗号在末尾的情况） #

如果确认逗号总是出现在字符串最后一位，可以直接通过切片去掉最后一个字符：

price_str = line[3]
if price_str[-1] == ',':
    price_str = price_str[:-1]
line[3] = float(price_str)

把上述任意一段代码替换原出错处的line[3]=float(line[3])即可完成转换。

内容的提问来源于stack exchange，提问作者cc cc