How come a file doesn't get written until I stop the program?

Writing to disk is slow, so many programs store up writes into large chunks which they write all-at-once. This is called buffering, and Python does it automatically when you open a file.

When you write to the file, you're actually writing to a "buffer" in memory. When it fills up, Python will automatically write it to disk. You can tell it "write everything in the buffer to disk now" with

f.flush()

This isn't quite the whole story, because the operating system will probably buffer writes as well. You can tell it to write the buffer of the file with

os.fsync(f.fileno())

Finally, you can tell Python not to buffer a particular file with open(f, "w", 0) or only to keep a 1-line buffer with open(f,"w", 1). Naturally, this will slow down all operations on that file, because writes are slow.

You need to f.close() to flush the file write buffer out to the file. Or in your case you might just want to do a f.flush(); os.fsync(); so you can keep looping with the opened file handle.

Don't forget to import os.

You have to force the write, so I i use the following lines to make sure a file is written:

# Two commands together force the OS to store the file buffer to disc
    f.flush()
    os.fsync(f.fileno())

You will want to check out file.flush() - although take note that this might not write the data to disk, to quote:

Note: flush() does not necessarily write the file’s data to disk. Use flush() followed by os.fsync() to ensure this behavior.

Closing the file (file.close()) will also ensure that the data is written - using with will do this implicitly, and is generally a better choice for more readability and clarity - not to mention solving other potential problems.