Add header to CSV files only once using Python - that are creating live

Question

So I am getting live JSON data from the stock market and creating CSV files correspondingly, for each csv file we need to add header.

def data_csv(self, task):
    ExchangeInstrumentName = task["Source"]
    fields = ["Time", "LastRate", "Volume", "AvgRate", "High", "Low", "Source"]
    csv_file = open(f"data/{ExchangeInstrumentName}.csv", "a")
    csv_writer = csv.DictWriter(csv_file, delimiter=',', fieldnames=fields)
    csv_writer.writerow(task)
    # csv_writer.writeheader()
    csv_file.close()

This is the function I am using to create the CSV files. fields is the variable in which my headers are stored. If I use writeheader() - for each row the headers are getting created.

I only want header at the top of the file. Please help

Answer 1

You could test the file size before adding a row. If it is zero bytes it will need a header. If it does not exist, the call will fail (and so will need a header).

For example:

import os


def data_csv(self, task):
    ExchangeInstrumentName = task["Source"]
    fields = ["Time", "LastRate", "Volume", "AvgRate", "High", "Low", "Source"]

    filename = f"data/{ExchangeInstrumentName}.csv"
    add_header = True

    try:
        # > than 0 bytes?
        if os.path.getsize(filename):   
            add_header = False
    except FileNotFoundError:
        pass    # does not exist    

    with open(filename, "a") as csv_file:
        csv_writer = csv.DictWriter(csv_file, delimiter=',', fieldnames=fields)
         
        if add_header:
            csv_writer.writeheader()
        
        csv_writer.writerow(task)

Using a with statement will ensure the file is automatically closed afterwards.