Using "with open(filename, 'ab'):" and calling code only if the file is new?

[Joseph L. Casale]

>     with open(self.full_path, 'r') as input, open(self.output_csv, 'ab') as
> output:
>         fieldnames = (...)
>         csv_writer = DictWriter(output, filednames)
>         # Call csv_writer.writeheader() if file is new.
>         csv_writer.writerows(my_dict)
> 
> I'm wondering what's the best way of calling writeheader() only if the file is
> new?
> 
> My understanding is that I don't want to use os.path.exist(), since that opens
> me up to race conditions.

What stops you from checking before and setting a flag?