How often does python flush to a file?


  1. How often does Python flush to a file?
  2. How often does Python flush to stdout?

I'm unsure about (1).

As for (2), I believe Python flushes to stdout after every new line. But, if you overload stdout to be to a file, does it flush as often?

5/7/2015 6:42:44 AM

Accepted Answer

For file operations, Python uses the operating system's default buffering unless you configure it do otherwise. You can specify a buffer size, unbuffered, or line buffered.

For example, the open function takes a buffer size argument.

"The optional buffering argument specifies the file’s desired buffer size:"

  • 0 means unbuffered,
  • 1 means line buffered,
  • any other positive value means use a buffer of (approximately) that size.
  • A negative buffering means to use the system default, which is usually line buffered for tty devices and fully buffered for other files.
  • If omitted, the system default is used.


bufsize = 0
f = open('file.txt', 'w', buffering=bufsize)
9/6/2018 10:12:52 PM

You can also force flush the buffer to a file programmatically with the flush() method.

with open('out.log', 'w+') as f:
    f.write('output is ')
    # some work
    s = 'OK.'
    # some other work

I have found this useful when tailing an output file with tail -f.

Licensed under: CC-BY-SA with attribution
Not affiliated with: Stack Overflow