How to rewrite only one frame (line[2]) in line in .csv file?

Question

I would like to rewrite value in csv file only in third frame for every line. First two frames would be same.

But in output of this code is

builtins.TypeError: '_csv.writer' object is not iterable

import csv

with open('friends.csv', mode='w') as file: 
    writer = csv.writer(file, delimiter=';', quotechar='"', quoting=csv.QUOTE_MINIMAL)
   
new_age = "25"

for line in writer:
    writer.writerow(['line[0]','line[1]',new_age]) #it means that first two frmaes will be rewrited for same value like they are, last one will be rewrited to new_age.

Do you know how makes it better?

Thank you

Does this answer your question? [Inline CSV File Editing with Python](https://stackoverflow.com/questions/16020858/inline-csv-file-editing-with-python) — Adam.Er8, Oct 27 '21 at 14:03
What is the rationale for the quotes around `line[0]` and `line[1]`??? — Serge Ballesta, Oct 27 '21 at 14:03
@SergeBallesta Yes, the shouldn´t be there, but it isn´t solving of my problem. — Mirek6, Oct 27 '21 at 14:14
@Adam.Er8 I think that is isn´t same. I need to rewrite some value. — Mirek6, Oct 27 '21 at 14:22

score 1 · Answer 1 · answered Oct 27 '21 at 14:29

1

A csv file is a text file. That means that the blessed way to change it is to write a different file and rename the new file with the original name in the end:

# open the files
with open('friends.csv', mode='r') as oldfile, open(
        'friends.tmp', mode='w', newline='') as newfile:
    # define a reader and a writer
    reader = csv.reader(oldfile, delimiter=';', quotechar='"')
    writer = csv.writer(newfile, delimiter=';', quotechar='"',
                        quoting=csv.QUOTE_MINIMAL)
    # copy everything changing the third field
    for row in reader:
        writer.writerow([row[0], row[1], ,new_age])
# ok, time to rename the file
os.replace('friends.tmp', 'friends.csv')

answered Oct 27 '21 at 14:29

Serge Ballesta

143,923
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122
252

Thank you. It is working but do you know some method to edit current file and not create new one? – Mirek6 Oct 27 '21 at 14:51
When I want at first print all rows ( for line in reader: print(';'.join(line))). All values from the file are deleted. – Mirek6 Oct 27 '21 at 15:33
@Mirek6: no because a text file is purely sequential. You can only safely edit in place file when you have fixed size records that can be separately modified. A good old text editor like notepad or vi always write a new copy when you change a single line... – Serge Ballesta Oct 27 '21 at 15:33
@Mirek6: you can only read a file up to its end once... You should either print **and** use `row` in the same (single) pass or rewind the file after the first full scan. – Serge Ballesta Oct 27 '21 at 15:35

score 0 · Answer 2 · answered Oct 27 '21 at 14:06

0

I think you need to Define your CSV file CSV_File = 'friends.csv

then call with open(CSV_file,'wb') as file: and you need to remove the quotes surrounding your line[1] line[0]

to skip first two lines open outfile with mode wb instead of w

answered Oct 27 '21 at 14:06

kamel jlassi

24
3

But what I should write instead line[1] and line[0]? I need to mark that I want to rewrite third frame and first two let how they are. – Mirek6 Oct 27 '21 at 14:13
You have to add newline="" as stated in the documentation if you're using python 3 as for python 2 use "wb" – kamel jlassi Oct 27 '21 at 14:17
Sorry but I don´t know where do you think I should put the newline="". – Mirek6 Oct 27 '21 at 14:21
with open('output.csv', 'wb', newline='') only if ure using python 3 – kamel jlassi Oct 27 '21 at 14:24
I have there: _builtins.ValueError: binary mode doesn't take a newline argument_ :-( – Mirek6 Oct 27 '21 at 14:28

score 0 · Answer 3 · answered Oct 27 '21 at 15:03

Have a look at convtools library, which provides both a csv helper and a lot of data processing primitives.

from convtools import conversion as c
from convtools.contrib.tables import Table

# if there's header and "age" is the column name
Table.from_csv("friends.csv", header=True).update(
    age=25
    # to reduce current ages you could:
    # age=c.col("age").as_type(int) - 10
).into_csv("processed_friends.csv")

# if there's no header and a column to rewrite is the 2nd one
Table.from_csv("friends.csv").update(COLUMN_1=25).into_csv(
    "friends_processed.csv", include_header=False
)

# if replacing the current file is absolutely needed
rows = list(
    Table.from_csv("friends.csv", header=True)
    .update(age=25)
    .into_iter_rows(list, include_header=True)
)
Table.from_rows(rows).into_csv("friends.csv", include_header=False)

How to rewrite only one frame (line[2]) in line in .csv file?

3 Answers3