Save to Text File from Infinite While Loop

Question

I am currently writing data from an infinite while loop to an SD Card on a raspberry pi.

file = open("file.txt", "w")
while True:
    file.write( DATA )

It seems that sometimes file.txt doesn't always save if the program isn't closed through either a command or a keyboard interrupt. Is there a periodic way to save and make sure the data is being saved? I was considering using

open("file.txt", "a")

to append to file and periodically closing the txt file and opening it up again. WOuld there be a better way to safely store data while running through an infinite while loop?

Answer 1

A file's write() method doesn't necessarily write the data to disk. You have to call the flush() method to ensure this happens...

file = open("file.txt", "w")
while True:
    file.write( DATA )
    file.flush()

Don't worry about the reference to os.fsync() - the OS will pretend the data has been written to disk even if it actually hasn't.

Answer 2

Use a with statement -- it will make sure that the file automatically closes!

with open("file.txt", "w") as myFile:
    myFile.write(DATA)

Essentially, what the with statement will do in this case is this:

try:
    myFile = open("file.txt", "w") 
    do_stuff()

finally:
    myFile.close()

assuring you that the file will be closed, and that the information written to the file will be saved.

More information about the with statement can be found here: PEP 343

Answer 3

If you're exiting the program abnormally, then you should expect that sometimes the file won't be closed properly.

Opening and closing the file after each write won't do it, since there's still a chance that you'll interrupt the program while the file is open.

The equivalent of the CTRL-C method of exiting the program is low-level. It's like, "Get out now, there's a fire, save yourself" and the program leaves itself hanging.

If you want a clean close to your file, then put the interrupt statement in your code. That way you can handle the close gracefully.