This makes no sense to me. Why would you write it in the first place if you are going to modify later? Why not write it correctly in the first place? Exactly what are you trying to achieve? What is the problem with the file that requires you to write it twice? Thanks

The new first row has only 2 features while the folowing rows have 30 features.

The pandas .to_csv function adds separators at the first row.

I need a fisrt row like this

"2";"tot"

and not like this

"2";"tot";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Again you have not explined what your requirement is just what you think it should be done. I can't think of a reason as to why you would want to have a custom format CSV file. It is normal for empty columns to show like that, otherwise how would you know which column is which piece of data? In your sample the first two columns are fine the rest are empty but what if the empty columns are at the middle of the row?

it is a specific format intended to be integrated into other software.

I don't know why this format is like this

Then scrap what you got and write the file correctly in the first place. Here is how you can iterate rows of a dataframe:

for index, row in df.iterrows():
    print(row['c1'], row['c2'])

Thank you for your help,

I don't know how to directly write a csv in the output folder of the recipe without using the function df.to_csv().

can you help me again ?

What is exactly the format of the file you want? What should be in the first row, second row, third row, etc.