Calling a subprocess with mixed data type arguments in Python

Question

I am a bit confused as to how to get this done.

What I need to do is call an external command, from within a Python script, that takes as input several arguments, and a file name.

Let's call the executable that I am calling "prog", the input file "file", so the command line (in Bash terminal) looks like this:

$ prog --{arg1} {arg2} < {file}

In the above {arg1} is a string, and {arg2} is an integer.

If I use the following:

#!/usr/bin/python
import subprocess as sbp 
sbp.call(["prog","--{arg1}","{arg2}","<","{file}"])

The result is an error output from "prog", where it claims that the input is missing {arg2}

The following produces an interesting error:

#!/usr/bin/python
import subprocess as sbp 
sbp.call(["prog","--{arg1} {arg2} < {file}"])

all the spaces seem to have been removed from the second string, and equal sign appended at the very end:

command not found --{arg1}{arg2}<{file}=

None of this behavior seems to make any sense to me, and there isn't much that one can go by from the Python man pages found online. Please note that replacing sbp.call with sbp.Popen does not fix the problem.

Answer 1

The issue is that < {file} isn’t actually an argument to the program, but is syntax for the shell to set up redirection. You can tell Python to use the shell, or you can setup the redirection yourself.

from subprocess import *

# have shell interpret redirection
check_call('wc -l < /etc/hosts', shell=True)

# set up redirection in Python
with open('/etc/hosts', 'r') as f:
    check_call(['wc', '-l'], stdin=f.fileno())

The advantage of the first method is that it’s faster and easier to type. There are a lot of disadvantages, though: it’s potentially slower since you’re launching a shell; it’s potentially non-portable because it depends on the operating system shell’s syntax; and it can easily break when there are spaces or other special characters in filenames.

So the second method is preferred.