How to debug subprocess call in Python/Django

Question

In my Python/Django code I call gdalbuildvrt process. This process should create a .VRT file, but it does not. In order to check it, I write the subprocess output to a debug file. This is how I do it:

process = subprocess.Popen(["gdalbuildvrt", mosaic, folder], stdout=subprocess.PIPE)
stdout = process.communicate()[0]

with open(os.path.join(os.path.dirname(os.path.abspath(__file__)), "debug.txt"), 'w') as file:
    file.write('{}'.format(stdout) + " -> " + mosaic)

As a result I see this in debug.txt file:

b'0...10...20...30...40...50...60...70...80...90...100 - done.\n' -> /var/www/vwrapper/accent/accent/layers/raw/mosaic.vrt

So, as you can see the first part of debug statement says, that it'ok:

0...10...20...30...40...50...60...70...80...90...100 - done.

And the second part says, that /var/www/vwrapper/accent/accent/layers/raw/mosaic.vrt should be created. However, when I go to the target folder and refresh it, I see no mosaic.vrt file there. So, what may be wrong with that and how can I fix it? I should add that on Windows machine it's 100% ok, but on CentOS it does not.

Log stderr and the .returncode... that's where any problems will be. — TemporalWolf
– TemporalWolf, Commented Mar 24, 2017 at 20:18
@TemporalWolf. Would you be so kind to elaborate on this a little bit? You may make a complete answer, if you wish. — Jacobian
– Jacobian, Commented Mar 24, 2017 at 20:20

TemporalWolf · Accepted Answer · 2017-03-24 20:28:43Z

4

process = subprocess.Popen(["gdalbuildvrt", mosaic, folder], 
                           stdout=subprocess.PIPE, stderr=subprocess.PIPE)
stdout, stderr = process.communicate()
ret = process.returncode

or

process = subprocess.Popen(["gdalbuildvrt", mosaic, folder], 
                           stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

which will redirect stderr to stdout.

Then log those two things:

All error logging should be on stderr, not stdout. And any return code will appear via process.returncode.

You could also probably use one of the higher processes, like subprocess.check_call()

edited Mar 24, 2017 at 20:28

answered Mar 24, 2017 at 20:23

TemporalWolf

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3 Comments

Jacobian Over a year ago

Thanks! I've just checked it. stderr gets None value and ret gets 0 value. It seems like there are no errors, but still process does not do, what it should do. I really wonder, what may be wrong.

TemporalWolf Over a year ago

@Jacobian I made an edit. you have to make sure you add a pipe for stderr or redirect it to stdout

Jacobian Over a year ago

Fantastic! Now I see where the error was. Thank you so much!

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