Question

In python I’m doing a os.system('chmod o+w filename.png') command so I can overwrite the file with pngcrush.

These are the permissions after I set them in python:

-rw-rw-rw- 1 me users 925 Sep 20 11:25 filename.png

Then I attempt:

os.system('pngcrush filename.png filename.png')

which is supposed to overwrite the file, but I get:

Cannot overwrite input file filename.png

What could be the problem? Isn’t pngcrush being run as an ‘other’ user, for which write permissions are enabled?

Thanks!

Answer 1

The problem is with the way you execute the pngcrush program, not with permissions of filename.png or Python. It simply attempts to open filename.png both for input and output, which is of course invalid.

Give pngcrush either the -e or the -d option to tell it how to write output. Read its man for more information.