old_contents: import random
  
  for i in range(0, 1000):
    y = random.sample(['0', '1'], 1)[0]
    x = []
    for j in range(0, 100):
      x.append(random.sample(['A', 'C', 'G', 'T'], 1)[0])
    x = ''.join(x)
    print ','.join([y, x])
  
new_contents: import random
  
  for i in range(0, 500):
    y = random.sample(['0', '1'], 1)[0]
    x = []
    for j in range(0, 1000):
      x.append(random.sample(['A', 'C', 'G', 'T'], 1)[0])
    x = ''.join(x)
    print(','.join([y, x]))
  
subject: Update generate to python3.