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?

CodePudding user response：

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):

1. open a new file for writing
2. read the first row of your CSV
3. interpret the column indexes as the header
4. write the header
5. write the first row
6. read/write the rest of the rows
7. 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)