Python's subprocess.Popen returns the same stdout even though it shouldn't

Question

I'm having a very strange issue with Python's subprocess.Popen. I'm using it to call several times an external exe and keep the output in a list.

Every time you call this external exe, it will return a different string. However, if I call it several times using Popen, it will always return the SAME string. =:-O

It looks like Popen is returning always the same value from stdout, without recalling the exe. Maybe doing some sort of caching without actually calling again the exe.

This is my code:

def get_key():

    from subprocess import Popen, PIPE

    args = [C_KEY_MAKER, '/26', USER_NAME, ENCRYPTION_TEMPLATE, '0', ]
    process = Popen(args, stdout=PIPE)
    output = process.communicate()[0].strip()
    return output

if __name__ == '__main__':
    print get_key() # Returns a certain string
    print get_key() # Should return another string, but returns the same!

What on Earth am I doing wrong?!

When you run the C_KEY_MAKER command twice -- directly on the command line -- what does it produce? — S.Lott
– S.Lott, Commented Apr 4, 2009 at 13:57
@Fernando: Actually not expected by me. I have my doubts that the command-line app has appropriate random number seeds. There must be some environment variable difference between the subprocess and the command-line version. — S.Lott
– S.Lott, Commented Apr 4, 2009 at 16:26

Greg · Accepted Answer · 2009-07-14 11:08:41Z

3

It is possible (if C_KEY_MAKER's random behaviour is based on the current time in seconds, or similar) that when you run it twice on the command line, the time has changed in between runs and so you get a different output, but when python runs it, it runs it twice in such quick succession that the time hasn't changed and so it returns the same value twice in a row.

answered Jul 14, 2009 at 11:08

Greg

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Comments

Devin Jeanpierre · Accepted Answer · 2009-04-04 13:59:45Z

1

Nothing. That works fine, on my own tests (aside from your indentation error at the bottom). The problem is either in your exe. or elsewhere.

To clarify, I created a python program tfile.py

cat > tfile.py
#!/usr/bin/env python
import random
print random.random()

And then altered tthe program to get rid of the indentation problem at the bottom, and to call tfile.py . It did give two different results.

answered Apr 4, 2009 at 13:59

Devin Jeanpierre

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2 Comments

cfischer Over a year ago

I'm running on Windows, could that be the problem?

Devin Jeanpierre Over a year ago

It's possible, but I doubt it. You could run such an example yourself and see if anything weird crops up, but that would be a serious bug in the subprocess module-- it should work identically in Linux and Windows, and it should work the way it does on my machine.

tzot · Accepted Answer · 2009-04-04 23:38:53Z

1

I don't know what is going wrong with your example, I cannot replicate this behaviour, however try a more by-the-book approach:

def get_key():

    from subprocess import Popen, PIPE

    args = [C_KEY_MAKER, '/26', USER_NAME, ENCRYPTION_TEMPLATE, '0', ]
    output = Popen(args, stdout=PIPE).stdout
    data = output.read().strip()
    output.close()
    return data

answered Apr 4, 2009 at 23:38

tzot

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Comments

Thomas Vander Stichele · Accepted Answer · 2009-04-04 14:10:51Z

0

Your code is not executable as is so it's hard to help you out much. Consider fixing indentation and syntax and making it self-contained, so that we can give it a try.

On Linux, it seems to work fine according to Devin Jeanpierre.

answered Apr 4, 2009 at 14:10

Thomas Vander Stichele

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