Writing list to a file in python?

Question

I have a product file which is in the following format:

12345670:Cadbury:0.50:100
12647859:Chips:1.50:50
16728769:Crisps:1.00:60
11111115:Olives:1.50:100
22222220:Blackberries:1.00:100
30298712:Gluestick:1.99:50
19832876:Cake:2.00:50
14054310:Phone:70.50:5
19208114:Banana:0.50:75
10928738:Raisins:0.75:100

Where the first part is the product code. The second is the Description. The third is the price and the fourth is the stock level.

The program will ask the user for a quantity of the product they want after they type in the product code. But how do I update the stock in the product file. Whenever I try it, it overwrites my entire file with a blank space. Please help

Answer 1

You're not explaining properly, and your question frankly is not an explicit question, but a code writing request (which is against StackOverflow's policy). So here is what you get:

contents with include a list of every line, each row of this list will include 4 lists separating your data.

with open('file.txt', 'r') as file:
    contents = [line.split(':') for line in file]

then you can modify (contents[index_num] = altered_value) or add (contents.append(new_value)) whatever it is you want to modify or add, and write it back to file, like so:

with open('file.txt', 'w') as file:
    for item in contents:
        print(item, file=file)

This is a quick fix!

Answer 2

You will need to read the whole file in, process it, and then write the whole file back. Modifying a file is not really possible.

This could be done as follows using Python's csv library to help parse the file, by specifying that the parameter delimiter is a :. By doing this, each entry is automatically read in as a list of parameters e.g. ['12647859', 'Chips', '1.50', '50']

import csv

stock_file = 'stock.csv'

stock = {}
with open(stock_file, 'rb') as f_stock:
    csv_stock = csv.reader(f_stock, delimiter=':')

    for cols in csv_stock:
        stock[cols[0]] = cols
    print stock

while True:
    product_code = raw_input("Please enter product code: ")
    product_quantity = int(raw_input("Please enter quantity: "))

    try:
        stock[product_code][3] = int(stock[product_code][3]) + product_quantity
        break
    except KeyError, e:
        print "Unknown product ID, try again"

with open(stock_file, 'wb') as f_stock:
    csv_stock = csv.writer(f_stock, delimiter=':')
    csv_stock.writerows(stock.values())

So for the following input:

Please enter product code: 12345670
Please enter quantity: 1000

The stock file would be updated as follows:

12647859:Chips:1.50:50
11111115:Olives:1.50:100
16728769:Crisps:1.00:60
10928738:Raisins:0.75:100
19208114:Banana:0.50:75
30298712:Gluestick:1.99:50
12345670:Cadbury:0.50:1100
22222220:Blackberries:1.00:100
19832876:Cake:2.00:50
14054310:Phone:70.50:5

Note, as the entries are stored in a dictionary, the ordering is not maintained in the file. If this is required, then it can be converted to use an OrderedDict

---

Note, if you are not allowed to use the csv library, it could be redone as follows:

stock_file = 'input.txt'

stock = {}
with open(stock_file, 'rb') as f_stock:
    for line in f_stock:
        cols = line.split(':')
        stock[cols[0]] = [col.strip() for col in cols]

while True:
    product_code = raw_input("Please enter product code: ")
    product_quantity = int(raw_input("Please enter quantity: "))

    try:
        stock[product_code][3] = str(int(stock[product_code][3]) + product_quantity)
        break
    except KeyError, e:
        print "Unknown product ID, try again"

with open(stock_file, 'wb') as f_stock:
    f_stock.write('\n'.join(':'.join(line) for line in stock.values()))