How to add a header to an existing CSV file without replacing the first row?

Question

What I want to do is actually as it is written in the title.

with open(path, "r+", newline='') as csv_file:
    csv_reader = csv.reader(csv_file, delimiter=',')
    list_of_column_names = []
    num_cols = len(next(csv_reader))
    for i in range(num_cols):
        list_of_column_names.append(i)
    fields = list_of_column_names
    with open(example.csv, "r+", newline='') as writeFile:
        csvwriter = csv.DictWriter(writeFile, delimiter=',', lineterminator='\n', fieldnames=fields)
        writeFile.seek(0, 0)
        csvwriter.writeheader()

I want to enumerate the columns which initially doesn't have any column names. But when I run the code, it replaces the data in the first row. For example:

example.csv:

a,b
c,d
e,f

what I want:

0,1
a,b
c,d
e,f

what happens after running the code:

0,1
c,d
e,f

Is there a way to prevent this from happening?

You'll have to overwrite the entire file, because there's no reasonable way to insert a line (i.e. row) into a text file because the length of each one varies. — martineau
– martineau, Commented Dec 31, 2021 at 18:52
Does this answer your question? Insert line at middle of file with Python? — Gwendal Delisle Arnold
– Gwendal Delisle Arnold, Commented Dec 31, 2021 at 19:09

Zach Young · Accepted Answer · 2021-12-31 19:41:30Z

1

There's no magical way to insert a line into an existing text file.

The following is how I think of doing this, and your code is already getting steps 2-4. Also, I wouldn't mess with the DictWriter since you're not trying to convert a Python dict to CSV (I can see you using it for writing the header, but that's easy enough to do with the regular reader/writer):

open a new file for writing
read the first row of your CSV
interpret the column indexes as the header
write the header
write the first row
read/write the rest of the rows
move the new file back to the old file, overwrite (not shown)

Here's what that looks like in code:

import csv

with open('output.csv', 'w', newline='') as out_f:
    writer = csv.writer(out_f)

    with open('input.csv', newline='') as in_f:
        reader = csv.reader(in_f)

        # Read the first row
        first_row = next(reader)
        # Count the columns in first row; equivalent to your `for i in range(len(first_row)): ...`
        header = [i for i, _ in enumerate(first_row)]

        # Write header and first row
        writer.writerow(header)
        writer.writerow(first_row)

        # Write rest of rows
        for row in reader:
            writer.writerow(row)

edited Dec 31, 2021 at 19:41

answered Dec 31, 2021 at 19:34

Zach Young

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6 Comments

chepner Over a year ago

This is a case where using enumerate is arguably less appropriate than using len and range. header = list(range(len(first_row))).

Bilal Sedef Over a year ago

Thank you! As you have guessed, I was using DictWriter for the header. Your solution works great!

Zach Young Over a year ago

@chepner, thanks for the suggestion! I disagree :) I copy-pasted your suggestion and looked at it, and... xrange of length of list materialized to a list just doesn't capture the idea for me. If I didn't use enumerate, I'd actually do it OP's way.

chepner Over a year ago

Or even just writer.writerow(range(len(first_row))), though creating a named list has value as documentation.

chepner Over a year ago

enumerate has to create tuples that are subsequently unpacked just to ignore the additional value that made tuples necessary to create in the first place.

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