如何附加文件并取出特定行python 3

Question

我有一个格式如下的股票文件：

12345678,Fridge,1,50
23456789,Car,2,50
34567890,TV,20,50

这是代码：

def main():
products = {}
#This is the file directory being made.
f = open('stockfile.txt') 
#This is my file being opened.

for line in f:


    # Need to strip to eliminate end of line character
    line = line[:-1]
    #This gets rid of the character which shows and end of line '\n'
    row = line.split(',')
    #The row is split by the comma
    products[row[0]] = [row[1], row[2],row[3]]
    #The products are equal to row 1 and row 2 and row 3. The GTIN is going to take the values of the product and price so GTIN 12345678 is going to correspond to Fridge and 1.

print(products)
total = 0

print('Id       Description         Total')
while True:
    GTIN = input('Please input GTIN ')
    if(GTIN not in products):
        print('Sorry your code was invalid, try again:')
        break

    row = products[GTIN]
    print(GTIN)
    description = row[0]
    value = row[1]
    stock = row[2]
    print(stock)

    quantity = input('Please also input your quantity required: ')
    row[2]= int(stock) - int(quantity)
    products[row[2]] = row[2]
    product_total= (int(quantity)*int(value))
    New_Stock  = GTIN + ',' + description + ',' + value + ',' + str(products[row[2]])
    f = open('stockfile.txt','r')
    lines = f.readlines()
    f.close()
    f = open("stockfile.txt","a")
    for row in lines:
        if((row + '\n') != (New_Stock + '\n')):
            f.write(New_Stock)
            f.close()

    print('%20s%20s%20s' % (GTIN, description, product_total))

    total = total + product_total

print('Total of the order is £%s' % total)
print(products)
main()

但是，代码不会更新股票。它应该做的是摆脱之前给定产品的库存，然后根据用户刚刚购买的数量进行更新。

我还没有开始，但是一旦库存达到零，我需要我的代码告诉用户我们已经缺货并需要一些新库存。然后需要向用户发送一条消息，等待我们补货，然后再显示补货价格。

如果您有时间，请您也编写这段新代码，如果没有，您能否解释一下如何更新库存以及为什么我的代码不起作用，谢谢。

Answer 1

当您seek 到给定行并调用write 时，为了完全覆盖该行，而不影响其他行或无意中创建新行，库存中的每行必须具有固定宽度。如何确保固定宽度？通过给记录中的每个字段一个固定的宽度。基本上，您选择每个字段可以包含的最大字符数；在这里，我假设所有字段为 8（尽管它们不能全部相同），因此您的库存将以这种方式存储：

12344848,  Fridge,       2,      50
13738389,      TV,       5,      70

如果你继续这样做，每行都会有一个最大宽度，这将使你能够找到行的开头并完全覆盖它。试试这个代码：

MAX_FIELD_LEN = 8

def main():
    products = {}
    product_location  = {}
    location = 0
    # This is the file directory being made.
    with open('stockfile.txt', 'r+') as f:
        # This is my file being opened.

        for line in f:
            # keep track of each products location in file  to overwrite with New_Stock
            product_location[line.split(',')[0]] = location
            location += len(line)
            # Need to strip to eliminate end of line character
            line = line[:-1]
            # This gets rid of the character which shows and end of line '\n'
            row = line.split(',')
            # The row is split by the comma
            products[row[0]] = [row[1], row[2], row[3]]
            # The products are equal to row 1 and row 2 and row 3. The GTIN is going to take the values of the product and price so GTIN 12345678 is going to correspond to Fridge and 1.

        print(products)
        total = 0

        while True:
            GTIN = input('Please input GTIN: ')
            # To terminate user input, they just need to press ENTER
            if GTIN == "":
                break
            if (GTIN not in products):
                print('Sorry your code was invalid, try again:')
                break

            row = products[GTIN]
            description, value, stock = row
            print('Stock data: ')
            print('GTIN \t\tDesc. \t\tStock \t\tValue')
            print(GTIN,'\t',description,'\t', stock, '\t', value)

            quantity = input('Please also input your quantity required: ')
            row[2] = str(int(stock) - int(quantity))
            product_total = int(quantity) * int(value)
            for i in range(len(row)):  
                row[i]  = row[i].rjust(MAX_FIELD_LEN)
            New_Stock = GTIN.rjust(MAX_FIELD_LEN) + ',' + ','.join(row) + '\n'
            #print(New_Stock, len(New_Stock))
            f.seek(product_location[GTIN])
            f.write(New_Stock)
            print('You bought: {0} {1} \nCost: {2}'.format(GTIN, description, product_total))

            total = total + product_total
        f.close()
        print('Total of the order is £%s' % total)

main()

使用此程序时，请确保 TXT 文件中的每个字段都正好是 8 个字符宽（不包括逗号）。如果要增加字段宽度，请相应地更改 MAX_FIELD_LEN 变量。您的 TXT 文件应如下所示：

Answer 2

在前几行中，您将整个数据文件加载到内存中：

for line in f:
    products[row[0]] = [row[1], row[2],row[3]]

那么只需更新内存中的数据，并让用户输入一个特殊命令：“save”将整个列表写入您的文件。

您还可以捕获您的应用程序进程 KILL 信号，因此如果用户按 ctrl + c，您可以在退出前询问他是否要保存。

并且可能每隔几秒钟将列表的临时副本保存到文件中。

Answer 3

如果您的客户打算多次运行此程序，我建议您为此使用shelve 模块。将整个文件读入内存并重新写入文本将随着库存的增长而变得低效。 Shelve 在您的 PC 上创建持久文件（确切地说是 3 个文件）以存储您的数据。最重要的是，shelve 将为您提供您想要的相同的dict 界面，因此您只需在文件上调用shelve.open()，您就可以开始使用 GTIN 作为键访问/更新您的股票。它非常简单，只需查看 python 手册即可。如果你真的想要一个文本文件，你可以让你的程序遍历包含你的股票的shelve文件（与字典相同）并将键（GTIN）及其值（你的股票数量）写入你的文本文件打开。通过这种方式，您可以轻松直观地访问您的记录，并在您的 TXT 文件中采用可读的格式。

Answer 4

MAX_FIELD_LEN = 8

def main():
    products = {}
    product_location  = {}
    location = 0
    # This is the file directory being made.
    with open('stockfile.txt', 'r+') as f:
    # This is my file being opened.

    for line in f:
        # keep track of each products location in file  to overwrite with New_Stock
        product_location[line.split(',')[0]] = location
        location += len(line)
        # Need to strip to eliminate end of line character
        line = line[:-1]
        # This gets rid of the character which shows and end of line '\n'
        row = line.split(',')
        # The row is split by the comma
        products[row[0]] = [row[1], row[2], row[3]]
        # The products are equal to row 1 and row 2 and row 3. The GTIN is going to take the values of the product and price so GTIN 12345678 is going to correspond to Fridge and 1.

    print(products)
    total = 0

    while True:
        GTIN = input('Please input GTIN: ')
        # To terminate user input, they just need to press ENTER
        if GTIN == "":
            break
        if (GTIN not in products):
            print('Sorry your code was invalid, try again:')
            break

        row = products[GTIN]
        description, value, stock = row
        print('Stock data: ')
        print('GTIN \t\tDesc. \t\tStock \t\tValue')
        print(GTIN,'\t',description,'\t', stock, '\t', value)

        quantity = input('Please also input your quantity required: ')
        row[2] = str(int(stock) - int(quantity))
        product_total = int(quantity) * int(value)
        for i in range(len(row)):  
            row[i]  = row[i].rjust(MAX_FIELD_LEN)
        New_Stock = GTIN.rjust(MAX_FIELD_LEN) + ',' + ','.join(row) + '\n'
        #print(New_Stock, len(New_Stock))
        f.seek(product_location[GTIN])
        f.write(New_Stock)
        print('You bought: {0} {1} \nCost: {2}'.format(GTIN, description, product_total))

        total = total + product_total
    f.close()
    print('Total of the order is £%s' % total)

main()

这是文本文件： 12345678, Fridge, 1, 50 23456789, Car, 2, 50 34567890, TV, 20, 50

我在 Mac 桌面上执行此操作或它是 python 3.4.3 是否有区别？

Answer 5

上面使用搁置的建议听起来是个好主意，但如果您想保持文件原样，但只使用（大部分）代码更新更改的记录（而不是每次都重写整个文件），这似乎有效。

def main():
    products = {}
    product_location  = {}
    location = 0
    # This is the file directory being made.
    with open('stockfile.txt', 'r+') as f:
        # This is my file being opened.

        for line in f:
            # keep track of each products location in file  to overwrite with New_Stock
            product_location[line.split(',')[0]] = location
            location += len(line)
            # Need to strip to eliminate end of line character
            line = line[:-1]
            # The row is split by the comma
            row = line.split(',')
            products[row[0]] = [row[1], row[2], row[3]]
            """
            The products are equal to row 1 and row 2 and row 3. The GTIN is going to take the values of the product and
            price so GTIN 12345678 is going to correspond to Fridge and 1.
            """

        print(sorted(products.items()))
        total = 0

        while True:
            GTIN = input('\nPlease input GTIN or press [Enter] to quit:\n')
            # To terminate user input, they just need to press ENTER
            if GTIN == "":
                break
            if (GTIN not in products):
                # Let the user continue with order after mistake in GTIN input
                print('Sorry your code was invalid, try again:')
                continue

            row = products[GTIN]
            print('GTIN:', GTIN)
            description = row[0]
            value = row[1]
            stock = row[2]
            stock_length = len(row[2])
            backorder = 0
            print('In Stock:', stock)

            quantity = input('Please also input your quantity required:\n')
            if int(quantity) > int(stock):
                row[2] = 0
                backorder = int(quantity) - int(stock)
                # TO DO
                Backordered_Stock = GTIN + ',' + description + ',' + value + ',' + str(backorder) + '\n'
            else:
                row[2] = int(stock) - int(quantity)
            products[row[2]] = row[2]
            product_total = (int(quantity) * int(value))
            New_Stock = GTIN + ',' + description + ',' + value + ',' + str(products[row[2]]).rjust(stock_length) + '\n'
            f.seek(product_location[GTIN])
            f.write(New_Stock)
            print('Ordered - {0:>6} GTIN: {1:>10}  Desc: {2:<20}  at £{3:>6}  Total value: £{4:>6}  On backorder: {5:>4}'.
                format(int(quantity), GTIN, description, int(value), product_total, backorder))

            total = total + product_total

        print('Total of the order is £%s' % total)

main()