Python: Adding element to list of a file without read and write each time

Asked Jul 29 '18 at 10:42

Active Jul 29 '18 at 10:46

Viewed 51 times

import json

# define list with values
basicList = [1, 2, 3, ..., 10000000]

# open output file for writing
with open('listfile.txt', 'w') as filehandle:  
    json.dump(basicList, filehandle)

I created a very long list and wrote it to a file. How can I now add a single element to the file without reading and rewriting the whole file?

edited Jul 29 '18 at 10:46

Martijn Pieters

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asked Jul 29 '18 at 10:42

Senju

You can't, not without careful re-writing of every value *after* the inserted value. – Martijn Pieters Jul 29 '18 at 10:44
use append open('listfile.txt', 'a') – aman5319 Jul 29 '18 at 10:45
Pick a different file format if that's an issue; JSON is not suited to incremental updates. – Martijn Pieters Jul 29 '18 at 10:45
@aman5319: that will **not** do what the OP wants. You can't just append to a JSON document, that breaks the JSON format. – Martijn Pieters Jul 29 '18 at 10:45
I am not dependant on JSON format. Can you provide a code example? – Senju Jul 29 '18 at 10:46
@MartijnPieters It's not a duplicate since this question contains the specific constraint of *not* reading / writing the whole file content each time an element is added. None of the answers addresses this issue. – a_guest Jul 29 '18 at 11:00
@a_guest: yes, it does cover that. Specifically, the [answer by SingleNegationElimination](https://stackoverflow.com/a/12994577/100297). – Martijn Pieters Jul 29 '18 at 12:18

Python: Adding element to list of a file without read and write each time

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