Output out of order when using system procedure

[Rock Nie]

I am running the system procedure in drracket's interactive window

(system "test.py")

where test.py is

#!/home/utils/Python-3.8.0/bin/python3

import os

print("Running cmd: date1")

print("Running cmd: date2")

os.system("date")

print("Running cmd: date3")

os.system("date")

and the output is

Tue May 19 22:14:19 PDT 2020

Tue May 19 22:14:19 PDT 2020

Running cmd: date1

Running cmd: date2

Running cmd: date3

#t

which is different from my expectation, can anyone help explain why?

[Rock Nie]

This only happens in DrRacket.

[Alex Harsanyi]

Short version: Try adding a "sys.stdout.flush()" after each of your print statements in your python script.

Longer version: when you run that program in a terminal, the output is a TTY and the standard output is flushed after every line.  When you run it as a subprocess, the standard output is a pipe and it is buffered.

This is not Racket specific, you would get the same problem if you ran "os.system("test.py")" in another python program.

Alex.

[Rock Nie]

Thx! sys.stdout.flush() works but in real use cases I cannot add sys.stdout.flush() to every python print.

Can I do anything in the racket side to solve the problem? Making the interactive window behave like terminal perhaps?

[Alex Harsanyi]

Change the first line of your python script to:

    #!/home/utils/Python-3.8.0/bin/python3 -u





Alex.