Question

From the python console this works:

convert -quality 100 in.pdf out.png

but when I add that command to my Python script like this:

Popen(['convert', '-quality 100', 'in.pdf', 'out.png'])

I get:

unrecognized option `-quality 100'

If I change that parameter to '-quality=100' I still get the error.

I tried fixing it like this:

Popen(['convert', '-quality', '100', 'in.pdf', 'out.png'])

which runs but fails to produce an out.png.

UPDATE: The last version is working. I must have mistyped it originally.

Answer 1

Every argument gets its own list element, so the second variant is correct.

You should bear in mind that until a call to communicate finishes, the command may still run (although that’s unlikely in your case). Check returncode after calling communicate to find out whether the program encountered any errors (like a malformed PDF file or so).

Also, imagemagick convert writes out multipage PDFs to multiple PNG files (out-0.png, out-1.png). Check whether those exist. Use -append to supress that behavior.

import subprocess
Popen = subprocess.Popen
s = Popen(['convert', '-quality', '100', 'in.pdf', '-append', 'out.png'])
s.communicate()
if s.returncode != 0:
   raise OSError('convert error')