Python write to txt file at specific line

Question

I am creating a python stock system in ,which I need to allow a user to enter a product code and enter their quantity and the software checks a txt file. It checks whether the product is found and whether their is enough of them. I am stuck trying to make my code subtract the quantity needed from the total quantity in the text file. I only want to change the quantity of that one product. Any help will be appreciated. Thanks Ben

Edit:

I have to use txt as it is part of the design brief.

x = 10
y = 0
n = 1
data = [line.strip().split() for line in open("Untitled.txt")]
code = input("Please enter Product Code?")
quantity = input("How many of these do you need?")


for num in range(0,10):
    if data[x][0] == code:
        n = 0
        y = x
    x = x - 1

if n == 0:
    if data[y][3] >= quantity:
        print("Product in stock")

        old = int(data [y][3])
        quantity = int(quantity)
        new = old - quantity
        new = str(new)
        data[y][3] = new
    else:
        print("Product out of stock")
else:
    print("Product not found")

This is my txt file

Code        Description Price   Quantity
12345670    testitem    1.45    34
12345671    testitem    1.45    34  
12345672    testitem    1.45    34
12345673    testitem    1.45    34
12345674    testitem    1.45    34
12345675    testitem    1.45    34
12345676    testitem    1.45    34
12345677    testitem    1.45    34
12345678    testitem    1.45    34
12345679    testitem    1.45    34
12345680    testitem    1.45    34

can I ask why are you saving it in text file? It is difficult to detect the column to write into and it is less error prone if you write it in a database — Moses Liao GZ, Feb 26 '16 at 07:54
@ben.garman, OK just do your operation in a sqlite bd and dump (cron) to the txt file. — Ali SAID OMAR, Feb 26 '16 at 07:57
@Ali SAID OMAR please explain what that means? (maybe post as an answer) — Tadhg McDonald-Jensen, Feb 26 '16 at 08:42

score 1 · Answer 1 · answered Feb 26 '16 at 09:08

tablib is an excellent tool for manipulating such data with you example (I changed the .txt by a csv file, tabulated separated)

test.csv

Code    Description Price   Quantity
12345670    testitem    1.45    34
12345671    testitem    1.45    34
12345672    testitem    1.45    34
12345673    testitem    1.45    34
12345674    testitem    1.45    34
12345675    testitem    1.45    34
12345676    testitem    1.45    34
12345677    testitem    1.45    34
12345678    testitem    1.45    34
12345679    testitem    1.45    34
12345680    testitem    1.45    34

stock.py

# coding=utf-8
#! /usr/bin/env python
from tablib import Dataset

# load data from the tsv
with open("./test.csv") as fin:
    imported_data = Dataset().load(fin.read())

code = input("Please enter Product Code?")
quantity = int(input("How many of these do you need?"))

# code is uniq find the first index
i = imported_data["Code"].index(code)
new_qt = int(imported_data[i][3]) - quantity
# tuple manipulation, so replace the line, item affection not possible
if new_qt >= 0:
    imported_data[i] = imported_data[i][:-1] + (str(new_qt),)
else:
    imported_data[i] = imported_data[i][:-1] + ("0",)

export = imported_data.tsv
# newline="" avoid add new lines 
with open("./test.csv", "w", newline="") as fout:
    fout.write(export)

wow I like that you can just do `imported_data["Code"]` to get the column as a list. I definitely will look into `tablib` but I still think it is a bit overkill for the question since it has no resemblance to the original code in question. — Tadhg McDonald-Jensen, Feb 27 '16 at 03:36

Tadhg McDonald-Jensen · Answer 2 · 2016-02-26T08:15:29.247

0

All you need to do is do is this in reverse:

data = [line.strip().split() for line in open("Untitled.txt")]

so each line in data joined by "\n" and each element in line joined by... a tab? (.split() without arguments could split by a few different characters)

new_text = "\n".join("\t".join(line) for line in data)

I see you are opening the file once for reading into data and one for r+ but it only needs to be opened for the line of code that it gets written to:

with open("Untitled.txt","w") as f:
    f.write(new_text)

Edit: just noticed you have a typo / mistake:

if data[y][3] ...:
    ...
    old = ...data [y][3]..
    ...
    data[1][3] = new

it is quite possible you just need to change the last 1 to y

edited Feb 26 '16 at 08:15

answered Feb 26 '16 at 08:05

Tadhg McDonald-Jensen

20,699
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No, I want to only replace the quantity – ben.garman Feb 26 '16 at 08:10
otherwise you already are replacing the quantity in `data`, all that is left is to rewrite the file I was guessing, maybe I was mistaken? – Tadhg McDonald-Jensen Feb 26 '16 at 08:23
I need to put another bit of code in to repeat it if they have more products. I only need to replace where it says the amount of quantity. – ben.garman Feb 26 '16 at 08:25
if you put in the `new_text` line and then `print(new_text)` does it not express the desired change? I really think you have correctly handled the changing the value part. – Tadhg McDonald-Jensen Feb 26 '16 at 08:34
Yes. It works but I need to know how to replace it in the txt file – ben.garman Feb 26 '16 at 08:36
put the `with open...` lines that @Adrien Chaussende copied from me at the bottom of file or in conditional that modifies data – Tadhg McDonald-Jensen Feb 26 '16 at 08:39

Adrien Chaussende · Answer 3 · 2017-02-06T14:05:58.137

If you are using Python2, you are compairing data[x][0] which type is str with potentially any kind of type for the input. Your code works well if the user types '12345670' (or any other product code between ''). In Python3, it is ok.

Some other points:

You are opening the file twice. Just do it once and use the variable in your code after the opening.
Your for block is quite complicated imo. I would do this like for x in range(1, len(data)) but if you are fine like this, it is working.
Last one. You are not writing into file after that. You must write your data to the file to update it.
True last one. Maybe you should use some database if you want to store, access, and update your data.

Edit

For the write part, here is your code edited (credits to Tadhg McDonald-Jensen)

x = 10
y = 0
n = 1
edit = open("data.txt","r+")
data = [line.strip().split() for line in open("data.txt")]
code = input("Please enter Product Code?")
quantity = input("How many of these do you need?")

for num in range(0,10):
    if data[x][0] == code:
        n = 0
        y = x
    x = x - 1

if n == 0:
    if data[y][3] >= quantity:
        print("Product in stock")

        old = int(data [y][3])
        quantity = int(quantity)
        new = old - quantity
        new = str(new)
        data[y][3] = new
        with open("data.txt", "w") as f:
            new_text = "\n".join("\t".join(line) for line in data)
            f.write(new_text)
    else:
        print("Product out of stock")
else:
    print("Product not found")

" But be careful. The first element i.e. the one at index 0 is the column titles." That's why in the code, x starts with 10 and decrements and stops at 1. — Lafexlos, Feb 26 '16 at 08:18
I have to use txt as it is part of the design brief but thanks I will try it later — ben.garman, Feb 26 '16 at 08:19
also `input` in python 3 works like `raw_input` in python 2, so your main point is invalid if @ben.garman is using python3 — Tadhg McDonald-Jensen, Feb 26 '16 at 08:21

score 0 · Answer 4 · answered Feb 26 '16 at 08:43

Short answer: DON'T.

Medium answer: You should never change anything in the middle of a text file.

Long answer: The only foolproof way to change a text file is to write a new copy of the file with the change and rename it at the end when everything has been correctly writen to disk. That's what text editors do.

In your case, as the text file seems properly formatted with all lines being same length, you could try to process it as if it was a binary direct access file. But it only works if all fields keep same length all the time. Here are some reasons that could make it wrong:

you are using unicode characters encoded in utf8 - e and é seem equally long, but in utf8 e is one single byte (code 0x65) while é needs two bytes (0xC3 0xA9)
someone could edit the file with a text editor and inadvertently removes some characters from a field for example writing 1.5 instead of 1.50 => line is now shorter
someone could edit the file with a text editor and adds inadvertently some white spaces at an end of line => line is now longer ... and you cannot see it when looking at it in a text editor!

TL/DR: if you are required to use a text file for persisting your data, you must:

read everything at program start
keep everything in memory
flush everything to the disk file at program end and provide a way for your users to save their work (a command Save)

Alternatively, you could use a direct access private binary file or a database (sqlite is nicely integrated in Python) and just provide a text import/export functionnality.

score 0 · Answer 5 · answered Apr 19 '16 at 03:40

Working code :

x = 10
y = 0
n = 1
data = [line.strip().split() for line in open("Untitled.txt")]
code = input("Please enter Product Code?")
quantity = input("How many of these do you need?")

for num in range(0,10):
    if data[x][0] == code:
        n = 0
        y = x
        print("Data ",data[x][0])
    x = x - 1

if n == 0:
    if data[y][3] >= quantity:
        print("Product in stock")

        old = int(data [y][3])
        quantity = int(quantity)
        new = old - quantity
        data[y][3] = str(new)
        print(data)
        file = open("Untitled.txt", "w")
        for item in data:
            print>>file, (', '.join(item))
        file.close();
    else:
        print("Product out of stock")
else:
    print("Product not found")

Python write to txt file at specific line

5 Answers5