Question

I’ve been running some python code on Windows which opens up another python program ‘main_plot.py’ using the following line…

subprocess.Popen(['C:\\python26\\python.exe ','main_plot.py','-n', str(number_of_cores),'-m', str(number_of_motors)])

I’ve tried to ssh into a Mac OS pc to run the same code but it doesn’t work because I think I need to change the path. This was my first guess…

subprocess.Popen(['python','main_plot.py','-n', str(number_of_cores),'-m', str(number_of_motors)])

but I get the error…
python: can’t open file ‘main_plot.py’: [Errno 2] No such file or directory

I’ve also tried

subprocess.Popen(['python','~/code/stochastic/main_plot.py','-n', str(number_of_cores),'-m', str(number_of_motors)])

but I get the same error.

I’ve checked that the file is in ‘~/code/stochastic’ and it is. I’m a bit stuck as what to do next

Answer 1

The problem isn’t a difference between Windows and Mac; it’s that you’re only using ~ on Mac, and you can’t use ~ in a pathname.

Put a different way, ~/code/stochastic/main_plot.py is not a real pathname—or, rather, it is, but it’s looking for a directory named ~ under the current directory, not your home directory. The shell turns that into the real pathname using tilde-expansion. Python can do tilde-expansion as well, but you have to ask it to do so explicitly.

So, the solution is simple:

subprocess.Popen(['python',
                  os.path.expanduser('~/code/stochastic/main_plot.py'),
                  '-n', str(number_of_cores),'-m', str(number_of_motors)])

As Jeremy Roman points out in a comment, you can use ~ in paths if you use shell=True, because then Python will put all of your args together into a command line to pass to the shell, and the shell does handle ~. But you don’t want to do that. Just call expanduser.

For future reference, the same thing is true for all the other kinds of expansion the shell does. You can’t do "${HOME}/foo", but you can do os.path.expandvars("${HOME}/foo"). You can’t do "foo$((1+1))bar"; you have to do something like "foo%sbar" % (1+1,). And so on.