I think you should reread the Popen docs carefully :)

That example should be:

call([sys.executable, 'diff.py', 'oldfile', 'newfile'])

And there are examples of that call format in the Popen docs, as well as the explanation of exactly how to format the list correctly.  So again, I don't think it is a good idea to try to duplicate that in the call/check_call docs.