-u标志对Python脚本的性能或执行有何作用？

Question

A multiprocessing.Process powered script I was writing was not printing to the terminal as expected, so instead of adding flush=True to every single print function, I added the -u flag to my python command line.

So instead of this:

python /path/to/script

I ran:

python -u /path/to/script

And voila! The myriad print functions all printed as expected.

What else does running a Python script unbuffered do besides making the print function work as expected? (is my main question...)

And why isn't it unbuffered in the first place?

Thank you!

Answer 1

Buffering improves performance when large amounts of output are being generated incrementally but don't need to be processed in real time.

For instance, if you are downloading data from a remote server and writing it to a file or the console, and all you really care is the end result (the complete file or the full output), then it's reasonable to not output a few bytes at a time to the file, but instead wait for a large chunk of bytes to be received and then write them all at once - less time spent in write function calls, and (if writing to disk) fewer disk seeks and more long continuous writes.

Normally, invocations of Python will flush their output when the python process exits and/or when the buffer is full. Chances are whatever you were doing either wasn't terminating gracefully or wasn't generating enough output to fill up the buffer, and therefore the normal conditions for flushing the buffer weren't being triggered.