python csv header ignore while keep appending data to csv file

Question

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 python csv, writing headers only once, but couldn't implement properly. If somebody help me to correct my code. I will be thankful.

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})

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.

Answer 1

A slightly simpler alternative to Mr Evans approach would be to use the following test in place of the test for existence:

fileEmpty = os.stat('collection1.dat').st_size == 0

This obviates the need to do a seek, etc.

EDIT: Complete code:

import random
import csv
import os.path
from time import gmtime, strftime

filename = '/home/robdata/collection1.dat'

fileEmpty = os.stat(filename).st_size == 0

v = random.randint(0, 100)

with open(filename, "a") as csvfile:
    headers = ['DATE', 'value']
    writer = csv.DictWriter(csvfile, delimiter=',', lineterminator='\n',fieldnames=headers)
    if fileEmpty:
        writer.writeheader()  # file doesn't exist yet, write a header

    writer.writerow({'DATE': strftime("%Y-%m-%d %H:%M:%S", gmtime()), 'value': v})