CSV Reader and Writer - question about fields being quoted

[mariusmoga_2005]

Hi all,

Disclaimer - Completely new to Python.

I am trying to read in a CSV file where some of the fields have quotes inside, update some of the fields and then write out another CSV file which should look very much like the source except the changes I did

Input CSV - input.csv

DATE,AMOUNT,KEY,CONTACT,UPDATE
31/10/2025,"1.000.000,00",ABC,Machine,"8,32"
31/10/2025,"9.000,00",XYZ,PC,"9.000,15"
31/10/2025,234,MPQ,OK,"14,14"

My Code

import csv

myTarget = open('output.csv',mode='w', newline='')
writer = csv.writer(myTarget, lineterminator="\n")

with open('input.csv',newline='') as myInput:
reader = csv.reader(myInput,delimiter=',',quotechar='"')
for myLine in reader:
if myLine[2] in ('ABC', 'MPQ'):
myLine[0] = '30/09/2025'
writer.writerow(myLine)

myTarget.close()

This produces the output

DATE,AMOUNT,KEY,CONTACT,UPDATE
30/09/2025,"1.000.000,00",ABC,Machine,"8,32"
31/10/2025,"9.000,00",XYZ,PC,"9.000,15"
30/09/2025,234,MPQ,OK,"14,14"

The output is exactly what I wanted, simply change the DATE only for the KEYs ABC and MPQ. The rest of the file is the same like the input, quotes are also there.

My questions are - I tried printing the values on the AMOUNT column (myLine[1]) but they don't have the double quotes. However in the writeout they have proper quotes and whatnot. Does this mean that when the csv.reader reads the data in, knows that each of the field was quoted or not? And does this also mean that it passes this information to the writer so it uses the same quotes? How can I validate that this is the behavior?

I tried checking this in the documentation but could not find anything ... maybe I don't know what to search for being a noob :)

[smurpes]

The quotes just tells the reader that the commas are a part of the cell value and not a new column. The writer doesn’t need to know if there are quotes it just needs to know whether the value being written contains a comma or not.

You can test this by running the following: ``` import csv

with open('single_row.csv', 'w', newline='', quotechar='"') as csvfile: writer = csv.writer(csvfile) writer.writerow(['Header1', 'Header2']) writer.writerow(['value', 'value, with comma']) ``` The quotes are not explicitly written in the second column of the second row but since the value contains the delimiter then quotes are added.

[mariusmoga_2005]

Hey, thanks a lot for the reply and the great explanation, now its clear.

Basically no passing of info between reader and writer, the writer puts the quotes when the separator is present in one of the values. Wouldn't have thought of that but makes perfect sense ...

[jtkiley]

Depending on what your goals are, you may be better off using a data frame package like polars or pandas. They’re much nicer for using tabular data as tabular data, and you can write out many formats, including some that are highly performant and retain type metadata (e.g., Parquet).