Python execution order using subprocess.call

Question

I have the following code in file abc.py:

import subprocess
def evaluate():
    for i in range(5):
        print("Printing", i)
        subprocess.call(['ls', '-lrt'])
        print("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~")
evaluate()

Now when I call using python abc.py > out.dat, the output file contains the result of 'ls -lrt' five times which is followed by printing statements in the code. Why is it happening so and what should I do if I need to get output as:

printing 0

(results of ls -lrt here)

~~~~~~~~~~~~~~~~~~~~~~~

printing 1

.
.
.

and so on..?

Thank you..

Alfe · Accepted Answer · 2017-03-07 10:12:32Z

2

You need to flush your stream before you call a subprocess:

import subprocess, sys
def evaluate():
    for i in range(5):
        print("Printing", i)
        sys.stdout.flush()
        subprocess.call(['ls', '-lrt'])
        print("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~")
evaluate()

Flushing takes automatically place line-by-line as long as you write to a terminal (without redirection). Then you do not notice the problem because the print() flushes at the newline.

As soon as you redirect your output to a file the print() statement notices this and flushes automatically only when a certain buffer is full (and at termination time).

The subprocess (ls) does the same but uses its own buffer. And it terminates first, so its output is in the file first.

edited Mar 7, 2017 at 10:12

answered Mar 7, 2017 at 9:55

Alfe

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3 Comments

nir0s Over a year ago

This will still printout all "Printing #" before. He expects that it will print 1, then the results, then print 2... etc. The indentation is wrong.

Alfe Over a year ago

It was wrong in the OP, then was wrongly fixed by another user, I relied on that version. Understood now, fixed both OP and my post. I hope now everything's aligned.

Harry Over a year ago

That worked perfectly, the indentation in my post was wrong, I have corrected it, but nevertheless, I got the idea. Thanks a lot Alfe

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