Append new line to CSV file

Question

I'm trying to append new data to a CSV file.

For example: I have 3 columns, namely, Quantity, Cost, Item.

And say, I have the following data:

Quantity = 20
Cost = 150
Item = Keyboard

Each time I use the code, it should create a new row for the data - the output would look like:

Quantity,Cost,Item 20,150,Keyboard 20,150,Keyboard

Instead, the following code only produces the following output each time I run it:

Quantity,Cost,Item 20,150,Keyboard

Code:

QUANTITY = 20 COST = 150 ITEM = "Keyboard" with open('orders.csv', 'w', newline='') as order_csv: order_csv_write = csv.writer(order_csv) order_csv_write.writerow( ["QUANTITY", "COST", "ITEM"]) with open('orders.csv', 'a', newline='') as order_csv: order_csv_append = writer(order_csv) order_csv_append.writerow([QUANTITY, COST, ITEM]) order_csv.close()

How do I go about it the right way?

Uh, the first open obviously overwrites any previous contents. — tripleee
– tripleee, Commented Feb 26, 2022 at 10:44
You open the output file in 'w' mode. This will create a new file. You then open it with 'a' mode to append. What you probably want to do is to check if the file exists. If it exists don't open in 'w' mode and write the column heading just move on to append mode — jackal
– jackal, Commented Feb 26, 2022 at 10:45

eccentricOrange · Accepted Answer · 2022-02-26 10:54:48Z

Each time you use the write w mode, it overwrites the file, deleting the old contents in the process.

It might make more sense to first check if the file exists, and write the header row only if it doesn't. Here's one way to do that, and it shows the behaviour required in your question:

import csv from pathlib import Path QUANTITY = 20 COST = 150 ITEM = "Keyboard" FILE_PATH = Path('orders.csv') if not FILE_PATH.exists(): with open(FILE_PATH, 'w', newline='') as order_csv: order_csv_write = csv.writer(order_csv) order_csv_write.writerow( ["QUANTITY", "COST", "ITEM"]) with open(FILE_PATH, 'a', newline='') as order_csv: order_csv_append = csv.writer(order_csv) order_csv_append.writerow([QUANTITY, COST, ITEM])

Additionally, you don't have to call order_csv.close() when using with ... as, as it's handled automatically.
That wasn't my code, and you're right - I knew it wasn't necessary. Thanks a ton!

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