如何在文本文件中编辑特定行的一部分

Question

I have a list of stock inventory. For example:

1003, table, 12.99, 19.99, 45

1004, chair, 3.99, 9.99, 32

where it is written in the format: Product code, name, costprice, price, quantity

How would I be able to update the quantity so when orders are placed - decreases by 1 etc?

This is the code I have so far:

def stockupdate():
itemname = input("Enter product name: ")
f = open('Stockinventory.txt', 'r') 
search = f.readlines() 
f.close() 
for line in search:
    lst = line.split(", ") #split 
    if str(itemname) == lst[1]: #Check to see if number entered is first parameter on list
        print ("SIN: %s" % lst[0])
        print ("Itemname: %s" % lst[1])
        print ("Retail price: %s" % lst[2])
        print ("Costprice: %s" % lst[3])
        print ("Quantity Available: %s" % lst[4])

This first section above just displays currently held product info to the user

choice = input("Would you like to update quantity? y/n")
if choice == 'y':
    newquantity = input("Enter new quantity: ")

    with open('Stockinventory.txt', 'r') as f:
        data = f.readlines()
    f.close()
    for line in data:
        lst = line.split(", ")
        if str(itemname) == lst[1]:
            lst[4] = str(newquantity)
            lst = line.join(", ")
            f = open('Stockinventory.txt', 'w')
            if itemname != lst[1]:
                f.write(line)
            else:
                f.write(lst)

At the moment I always end up deleting what is already in the text file

Thanks for your time, any help is appreciated

Answer 1

I would read it using the cvs module. then place all the data in either a dict or put it in a pandas dataframe,

In either, you can manipulate the content.

When finished, you can write your data back to disk

If you want to do it your way, you should open and close the file outside the loop, and write all lines, replacing the line you want altered. My experience is, that this method is too difficult, especially if you want to manipulate more complex items.