Python delete CSV row without creating a new file

Question

I am using Python (Raspberry Pi) to edit a CSV file. I open the file with the "append" tag and add a line. After that I check the size of the fil. If the file size is too big, I want to delete the first row of data (there is a headers row). Every example I see just skips the row and the writes all the other rows to a different CSV file. I do not want to have to create a new CSV file...I just want to delete the first row in the current file and save it.

import csv

def write_csv(datalist):   

    with open("CSVfile",'a') as f:
        writer = csv.writer(f, delimiter=',' ,quoting=csv.QUOTE_NONE)
        writer.writerow(datalist)

    while os.path.getsize("CSVfile") > 100000:
        ****DELETE FIRST ROW OF DATA FROM CSV FILE****

You will have to repeatedly save it again. Each time with the first row of datalist removed, until the size is below 100000. — totoro
– totoro, Commented Apr 15, 2016 at 18:57
This code has an error, there is no writeline for the csv.writer object. — David Zemens
– David Zemens, Commented Apr 15, 2016 at 18:59
@ThomasChristensen how would I code it so that I write to a new file each time...yet the file name stays "constant" so that the loop would work properly if it needed to cycle through a few times? — Crewski
– Crewski, Commented Apr 15, 2016 at 19:04
You need to use a second file. You can create a new file and write to it. When the write has completed successfully, you would then rename the temporary file to the original name. — dsh
– dsh, Commented Apr 15, 2016 at 19:07

totoro · Accepted Answer · 2016-04-15 19:56:55Z

0

Just a way to do it for inspiration. There is no need to make a new file and rename in this case:

import csv
import os
import random

def ensure_csv_max_size(size):
    # Read all data into memory.
    with open('CSVfile', 'r') as f:
        reader = csv.reader(f, delimiter=',', quoting=csv.QUOTE_NONE)
        csv_data = list(reader)

    # Remove first row, save, check repeatedly.
    for i in range(len(csv_data)):
        with open('CSVfile', 'w') as f:
            writer = csv.writer(f, delimiter=',', quoting=csv.QUOTE_NONE)
            writer.writerows(csv_data[i:])

        print(os.path.getsize('CSVfile'))
        if os.path.getsize('CSVfile') <= size:
            break

def write_csv(datalist):   

    with open("CSVfile",'a') as f:
        writer = csv.writer(f, delimiter=',' ,quoting=csv.QUOTE_NONE)
        writer.writerow(datalist)

    while os.path.getsize("CSVfile") > 100000:
        print("cut")
        ensure_csv_max_size(100000)

for i in range(1000):
    write_csv([random.randint(0, 100) for _ in range(10)])

edited Apr 15, 2016 at 19:56

answered Apr 15, 2016 at 19:20

totoro

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4 Comments

Antti Haapala Over a year ago

And if the system crashes you've lost the file; and in addition this will ruin the sd card in no time.

totoro Over a year ago

@AnttiHaapala All true.

Crewski Over a year ago

@totoro thank you for the example. I ended up doing as everyone suggested and writing a new file then renaming it to the original one. Didn't seem like the risk of corrupting things was worth it.

totoro Over a year ago

@Crewski Good idea. My answer was merely providing a generel example. I didn't have Raspberry Pi in mind.

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