Do you write Python (not very well?).
Ever wondered why your calls to os.execvp() and its ilk don’t work as expected?
The answer may be that you’re doing this:
os.execvp('ls', ['/home/fred'])
When you actually mean this:
os.execvp('ls', ['ls', '/home/fred'])
As ever, reading the Python docs (help(os.execvp)) is usually better than randomly googling for an answer.