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

[Larz60+]

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.
If for some obscure reason that didn't work, you can always close and then reopen.

I assume since cpython is written in C. Seek works the same.
So you should be able to move around the file at will. This doesn't make sense unless dealing with
fixed length records, unless you return to a previous position.
for example, use tell to get current position, seek to 0 to reread header, and then seek back to position saved from the tell