python-通过在每行中添加字符来编辑csv

Question

I've got this dataset in a .csv file: https://www.dropbox.com/s/2kzpzkhoiolhnlc/output.csv?dl=0

19,3,12
3
12
16,4
26,15,8,3
2
8
15
20
12,25,20,2,16
12,16
12,25
2,16
1,12
16,4
11,19,25,20
11,20,16,21
25,20,21
.....

for each row, if numbers are less then 51 than I need to add ? until having 51 chars in that row. For example, in the first row I have 19,3,12, so I have to add 48 ? to have a row like this: 19,3,12,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,? In the second row I have just one number, so I have to add 50 ?, the same for the other rows. Could you help me please?

EDIT: I've tried with this but didn't work, it just added "" to some rows:

import pandas as pd
df = pd.read_csv('output.csv', sep=';')
df = df.fillna('?')
df.to_csv('sorted2.csv', index=False)

Answer 1

you can do it with just text file manipulation if you want, no need to use pandas or csv module for this simple case.

import csv
with open('source.csv') as f:
    with open('result.csv', 'w') as fw:
        for line in f:
            line = line.strip() + (',?' * (50 - line.count(',')))
            fw.write(line + '\n')   

Answer 2

Use pandas to read the file in and set the number of columns you want. The following code reads in a file and assigns n columns. The extra elements will by default have value np.nan

df.read_csv('file', names=range(n))

If you want them to have a different value you can assign it with

df.fillna(value, inplace = True)

Then you can just write the dataframe back into the file and it will have the shape you want

df.to_csv('file')