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I'm trying to run a python script from a Windows shell (invoked via M-x shell or M-x powershell) inside Emacs.

However, no output is shown via print statements until I after I hit a raw_input statement AND provide input. In other words, the script shows no almost no output in the window until after I provide input, at which point all the output that should have been printed before get shown.

I say "almost" because LOGGER.info statements still get shown in my Emacs shell window, where LOGGER=logger.getLogger("loggerName")

For example if my script were...

LOGGER.info("Starting script")
print "Hello world."
raw_input("What's your name?").strip()

I would see

Starting script

And then nothing until I typed in something, at which point I would see:

Hello world.
What's your name?
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Perhaps buffering is at play and the logger implementation flushes the buffer. Try running with unbuffered stdin, stdout, stderr:

  • python -u yourscript.py

  • PYTHONUNBUFFERED=1 yourscript.py in your environment

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  • That worked! Thank you. Could I perhaps trouble you for an explanation of why that worked?
    – Anonymous
    Nov 30, 2017 at 16:42
  • 1
    Hmmm, I guess I would say that buffering is a way to optimize overall throughput at the cost of latency. For example, instead of issuing separate system calls for each character, you buffer them up into a larger chunks. Then a question is when to flush the buffer: size? time? special characters (eg newlines)? never and require the user/developer to explicitly flush the buffer? In your case, the print() isn't doing any explicit flushing but in newer pythons, there is a flush option you can use if you don't want to use something as blunt as PYTHONUNBUFFERED.
    – Erik Stephens
    Feb 27, 2018 at 22:19

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