subprocess: returncode v. poll()

[7stud]

Hi,

What is the difference between:

1) getting the returncode directly from the subprocess object
2) calling poll() on the subprocess object?

Here is an example:


import subprocess

p = subprocess.Popen("ls", stdout=subprocess.PIPE)
print p.returncode
print p.poll()
print

print p.stdout.read()[:5]
print

print p.returncode
print p.poll()
print p.returncode


--output:--
None
None

10tes

None
0
0

 

[7stud]

Hi,

What is the difference between:

1) getting the returncode directly from the subprocess object
2) calling poll() on the subprocess object?

Here is an example:

import subprocess

p = subprocess.Popen("ls", stdout=subprocess.PIPE)
print p.returncode
print p.poll()
print

print p.stdout.read()[:5]
print

print p.returncode
print p.poll()
print p.returncode

--output:--
None
None

10tes

None
0
0

Hmm....after a little more testing, I don't think returncode
dynamically updates:

import subprocess
import time

p = subprocess.Popen("ls", stdout=subprocess.PIPE)

print p.returncode
time.sleep(5)
print p.returncode
time.sleep(2)
print p.returncode

print p.stdout.read()[:5]
print p.returncode

--output:--
None
None
None
10tes
None

 

[7stud]

Hi,

What is the difference between:

1) getting the returncode directly from the subprocess object
2) calling poll() on the subprocess object?

Here is an example:

import subprocess

p = subprocess.Popen("ls", stdout=subprocess.PIPE)
print p.returncode
print p.poll()
print

print p.stdout.read()[:5]
print

print p.returncode
print p.poll()
print p.returncode


None
0
0

Hmm....after a little more testing, I don't think returncode
dynamically updates:

import subprocess
import time

p = subprocess.Popen("ls", stdout=subprocess.PIPE)

print p.returncode
time.sleep(5)
print p.returncode
time.sleep(2)
print p.returncode

print p.stdout.read()[:5]
print p.returncode

--output:--
None
None
None
10tes
None

....but then when is p.returncode set? And what good is it?

 

[David]

Hi,

What is the difference between:

1) getting the returncode directly from the subprocess object
2) calling poll() on the subprocess object?

Here is an example:

import subprocess

p = subprocess.Popen("ls", stdout=subprocess.PIPE)
print p.returncode
print p.poll()
print

print p.stdout.read()[:5]
print

print p.returncode
print p.poll()
print p.returncode


None
0
0

Hmm....after a little more testing, I don't think returncode
dynamically updates:

import subprocess
import time

p = subprocess.Popen("ls", stdout=subprocess.PIPE)

print p.returncode
time.sleep(5)
print p.returncode
time.sleep(2)
print p.returncode

print p.stdout.read()[:5]
print p.returncode

--output:--
None
None
None
10tes
None

...but then when is p.returncode set? And what good is it?

AFAICT p.returncode is only set by Popen methods. It doesn't
automatically get set by the OS internals that manage the actual
process.

One place where it is useful is when using Popen's communicate()
method, which returns (stdout,stderr), but not the return code. Also
it lets you call poll() but not have to save poll()'s return value.

 

[7stud]

Hi,
What is the difference between:
1) getting the returncode directly from the subprocess object
2) calling poll() on the subprocess object?
Here is an example:
import subprocess
p = subprocess.Popen("ls", stdout=subprocess.PIPE)
print p.returncode
print p.poll()
print
print p.stdout.read()[:5]
print
print p.returncode
print p.poll()
print p.returncode
--output:--
None
None
10tes
None
0
0
Hmm....after a little more testing, I don't think returncode
dynamically updates:
import subprocess
import time
p = subprocess.Popen("ls", stdout=subprocess.PIPE)
print p.returncode
time.sleep(5)
print p.returncode
time.sleep(2)
print p.returncode
print p.stdout.read()[:5]
print p.returncode
--output:--
None
None
None
10tes
None

...but then when is p.returncode set? And what good is it?

AFAICT p.returncode is only set by Popen methods. It doesn't
automatically get set by the OS internals that manage the actual
process.

One place where it is useful is when using Popen's communicate()
method, which returns (stdout,stderr), but not the return code. Also
it lets you call poll() but not have to save poll()'s return value.

I didn't understand your post when I first read it, but after looking
at my output again, and rereading your post, I get it now. In case
someone else doesn't understand it: returncode is not set by the
child process--ever. return code starts off with a default value of
None, and it remains None until you call a method in the subprocess
module, like poll() or wait(). Those methods set and then return
returncode. As a result, if you want to know what the status of the
child process is, you have to call either poll() or wait().