bash - Passing input to an executable using Python subprocess module

Question

I have an input file called 0.in. To get the output I do ./a.out < 0.in in the Bash Shell.

Now, I have several such files (more than 500) and I want to automate this process using Python's subprocess module.

I tried doing this:

data=subprocess.Popen(['./a.out','< 0.in'],stdout=subprocess.PIPE,stdin=subprocess.PIPE).communicate()

Nothing was printed (data[0] was blank) when I ran this. What is the right method to do what I want to do?

Answer 1

Redirection using < is a shell feature, not a python feature.

There are two choices:

1. Use shell=True and let the shell handle redirection:

   data = subprocess.Popen(['./a.out < 0.in'], stdout=subprocess.PIPE, shell=True).communicate()
   

2. Let python handle redirection:

   with open('0.in') as f:
       data = subprocess.Popen(['./a.out'], stdout=subprocess.PIPE, stdin=f).communicate()
   

The second option is usually preferred because it avoids the vagaries of the shell.

If you want to capture stderr in data, then add stderr=subprocess.PIPE to the Popen command. Otherwise, stderr will appear on the terminal or wherever python's error messages are being sent.