title: Python CSV reader returns fewer rows than in file
body: <p>I am struggling with the <code>csv</code> module. I have a sample CSV file that has 5000 lines (each line contains 7 values 0 or 1) with headers. I want to iterate through file in read mode and append file in write mode with new column values (<code>prediction</code>), but iteration stop after 478th row (like in sample code):</p>
  
  <pre><code>import csv
  import random
  
  
  def input_to_csv():
  
      prediction = [round(random.uniform(0, 1), 0) for _ in range(1, 5000)]
  
      combined_set = list(map(str, prediction))
  
      export_columns = ['COLUMN ' + str(n) for n in range(1, 8)] + ['OUTPUT'] 
  
      rr = 0
      with open('test.csv', 'r') as input_file:
  
          csv_input = csv.reader(input_file)
          next(csv_input)
  
          with open('test.csv', 'w', newline='') as csv_file:
  
              writer = csv.writer(csv_file)
              writer.writerow(export_columns)
  
              for row in csv_input:
  
                  rr += 1
  
          print(rr)
  </code></pre>
  
  <p>I have checked length of the csv_input file using <code>row_count = sum(1 for _ in input_file)</code> which gave me 5000 lines.</p>