NoneType object is not Iterable when write into CSV file

Question

in my first Python program I try to find duplicate values in my list which is loaded from CSV file (over 98+ k (98 000 lines) of rows in CSV file each with 5 columns) and save into list like Object (I use only 2 columns and in CNT column I save number of duplicates values):

class Duplication:
  def __init__(self, pn, comp, cnt):
    self.pn = pn
    self.comp = comp
    self.cnt = cnt

  def __str__(self):
    return f'{self.pn};{self.comp};{self.cnt}\n'

  def __repr__(self):
    return str(self)

  def __hash__(self):
    return hash(('pn', self.pn,
             'competitor', self.comp))
  def __eq__(self, other):
    return self.pn == other.pn and self.comp == other.comp

After that I select only files which I had more times in list and try it to save duplicate object into new CSV file:

  results = [d for d in duplicates if d.cnt > 1]
  results = set(results) 

  with open(f'fileName.csv', 'a') as f:
        f.writelines('=== Info Duplications to Delete ===\n')
        for line in results:
            f.writelines(print(line))
        f.close()    
    print(results)

I got this error, but in results are over 7+ k values, which I want to save into CSV file when I have a smaller list under 100 values, data will be saved, but with this file with large data row.

I had this problem and I check data in the file and also in the debugger, and there is no None value or something which looks like a problem or invalid data

---

UPDATE

After change to:

  with open(f'file.csv', 'a') as f:
        f.writelines('===Info ===\n')
        f.writelines(results)
        #for line in results:
        #    f.writelines(print(line))
        f.close()    
    print(results)

I got this error:

Run this script take over 20 minutes

Answer 1

Ok, the solution was simple, I only change:

f.writelines(print(line))

to:

f.writelines(str(line)) 

Now everything works fine