Python subprocess: wait for command to finish before starting next one?

Question

I've written a Python script that downloads and converts many images, using wget and then ImageMagick via chainedsubprocess calls:

for img in images: 
  convert_str = 'wget -O  ./img/merchant/download.jpg %s; ' % img['url'] 
  convert_str += 'convert ./img/merchant/download.jpg -resize 110x110 ' 
  convert_str += ' -background white -gravity center -extent 110x110' 
  convert_str += ' ./img/thumbnails/%s.jpg' % img['id']
  subprocess.call(convert_str, shell=True)

If I run the content of convert_str manually at the command line, it appears to work without any errors, but if I run the script so it executes repeatedly, it sometimes gives me the following output:

--2013-06-19 04:01:50--  
http://www.lkbennett.com/medias/sys_master/8815507341342.jpg
Resolving www.lkbennett.com... 157.125.69.163
Connecting to www.lkbennett.com|157.125.69.163|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 22306 (22K) [image/jpeg]
Saving to: `/home/me/webapps/images/m/img/merchant/download.jpg'

 0K .......... .......... .                               100% 1.03M=0.02s

2013-06-19 04:01:50 (1.03 MB/s) - 
`/home/annaps/webapps/images/m/img/merchant/download.jpg' saved [22306/22306]

/home/annaps/webapps/images/m/img/merchant/download.jpg 
[Errno 2] No such file or directory: 
' /home/annaps/webapps/images/m/img/merchant/download.jpg'

Oddly, despite the No such file or directory message, the images generally seem to have downloaded and converted OK. But occasionally they look corrupt, with black stripes on them (even though I'm using the latest version of ImageMagick), which I assume is because they aren't completely downloaded before the command executes.

Is there any way I can say to Python or to subprocess: "don't run the second command until the first has definitely completed successfully?". I found this question but can't see a clear answer!

Answer 1

Normally, subprocess.call is blocking.

If you want non blocking behavior, you will use subprocess.Popen. In that case, you have to explicitly use Popen.wait to wait for the process to terminate.

See https://stackoverflow.com/a/2837319/2363712

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BTW, in shell, if you wish to chain process you should use && instead of ; -- thus preventing the second command to be launched if the first one failed. In addition, you should test the subprocess exit status in your Python program in order to determine if the command was successful or not.