Check existing values in text file before appending new values

Question

I'm trying to make a simple cache that stores values I've already generated so I don't append duplicate values into the text file. However, I've been having issues with this, no matter what I seem to try my program keeps appending duplicate values.

Here's my current code, if anyone could point me in the right direction it would be greatly appreciated:

import random

def test():
    cache = open('cache.txt','a+')

    for x in range(10):
        number = random.randint(1,20)
        if number not in cache:
            cache.write(str(number) + '\n')

    print("Finished.")

test()

related: [Python: Open a file, search then append, if not exist](http://stackoverflow.com/q/28385337/4279) — jfs, Feb 08 '15 at 18:53

Anshul Goyal · Accepted Answer · 2015-02-08T19:03:35.160

4

You will have to keep a track of existing values, before you can add new values. Also, in your current code, you check for the in membership test against a file object, which just won't work.

So instead, read the existing values into a variable first, and then add new values checking if they already exist or not. Also add the newly generated no duplicate random values into existing values.

import random

def test():
    with open('cache.txt','r') as cache:
        existing = [l.rstrip("\n") for l in cache.readlines()]

    with open('cache.txt', 'a+') as cache:
        for x in range(10):
            number = str(random.randint(1,20))
            if number not in existing:
                existing.append(number)
                cache.write(number + '\n')
    print("Finished.")

test()

edited Feb 08 '15 at 19:03

answered Feb 08 '15 at 18:40

Anshul Goyal

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This still results in duplicates for me, are you sure this is correct? – Vale Feb 08 '15 at 18:45
1

@Scarecrow I had forgotten to add the newly generate random number to existing, adding `existing.append(number)` fixed that. – Anshul Goyal Feb 08 '15 at 18:46
Already upvoted and waiting on the timer for best answer. Thanks again! – Vale Feb 08 '15 at 18:48

score 2 · Answer 2 · answered Feb 08 '15 at 18:40

2

When you call number not in cache, you are trying to see if number is in an open() object. Instead, read the values from cache first.

answered Feb 08 '15 at 18:40

A.J. Uppal

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score 1 · Answer 3 · answered Feb 08 '15 at 18:38

1

cache is a file, not a dictionary or list; you'll need to read in the values from cache before you can do a look up on them.

answered Feb 08 '15 at 18:38

Marcus Müller

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Check existing values in text file before appending new values

3 Answers3