how can i safely write to a file when using Flask?

Question

I need to write a method on a Flask server which will write part of the request to a log file. However, if I understand correctly, Flask is multi-threaded, and there's a good chance that writing to a file is not safe.

Admittedly, I am more or less new to Python and multi-threaded programming in general, so I need someone to somewhat hold my hand through this a little bit :)

My code so far (slightly modified, without the parts of that would get me in trouble for posting online)

@app.route('/store_test')
def store_test():
    now = str(time.time())
    ip_address = request.remote_addr
    agent = request.user_agent.string

    log_data = [now,ip_address,agent]
    log_data_string = "\t".join(log_data)

    filename = "log.dat"

    f = open(filename,'a')
    f.write(log_data_string + "\n")
    f.close()

    return 'OK'

I'm guessing I need to wrap some code around the open and close statements, but I have no real idea what and I'm reading the Threads and Processes chapter in "Python in a Nutshell" book, and it’s not really giving me much of an idea about how to actually use these methods.

Any help would be appreciated.

Answer 1

Use the logging module. More in general, you need to use a Lock

import threading

lock = threading.Lock()
...

with lock:
    #Open the file and write to it

Basically, this is what logging does as well. More precisely, the Handler-objects (that actually write to some output, e.g., a file) implement locking.

It is important that all processes use the same Lock object, instead of creating their own. Thus, you could put it on module-level or similar.