In CSV, how to write the header after writing the body?

[squenson]

As far as I know you cannot go back to the beginning of a file with the csv package
It's a file, so you should be able to use seek command to position to beginning.

Correct, you can use file.seek(0) to position yourself at the beginning of a file but it is not part of the csv package. Furthermore, the OP doesn't want to read from the beginning of the file but to write a longer header and definitively the csv package is not meant for that.