title: python csv header ignore while keep appending data to csv file
body: <p>I am trying to add some random data into text file and I am successful in that but I am facing problem with the header line. I want to add header line once and then every time I run my script , it should add just data into file and ignore the header line if exists. I tried something like this but I fail. I try to look at this example code in SO <a href="https://stackoverflow.com/questions/28325622/python-csv-writing-headers-only-once">python csv, writing headers only once</a>, but couldn't implement properly. 
  If somebody help me to correct my code. I will be thankful. </p>
  
  <pre><code>import random
  import csv
  import os.path
  from time import gmtime, strftime
  
  filename = '/home/robdata/collection1.dat'
  
  file_exists = os.path.isfile(filename)
  
  v = random.randint(0, 100)
  
  with open(filename, "a") as csvfile:
      headers = ['DATE', 'value']
      writer = csv.DictWriter(csvfile, delimiter=',', lineterminator='\n',fieldnames=headers)
      if not file_exists:
          writer.writeheader()  # file doesn't exist yet, write a header
  
      writer.writerow({'DATE': strftime("%Y-%m-%d %H:%M:%S", gmtime()), 'value': v})
  </code></pre>
  
  <p>it insert data but without adding any header line. I want to include headers on the first run of script, and next time when I run script several times , it should only add data and not header line.  thank a lot for any idea or help. </p>