python subprocess grep returns zero

Question

I'm trying to count the number of times the string OW appears in a file with the following scrip,

import subprocess
subprocess.call("grewp Ow file.txt | wc -l", shell=True)

but it always returns the correct answer followed by zero

>>> subprocess.call("grep OW production_run.gro | wc -l", shell=True)
2638
0
>>> 

and when I try to declare a variable with it, it stores 0. Does anyone here have any idea of why that's happening and how to fix it?

Answer 1

Python's docs about subprocess.call

Run the command described by args. Wait for command to complete, then return the returncode attribute.

Try check_output instead

Answer 2

based on document subprocess.check_output :

Run command with arguments and return its output as a byte string.

Thus you can use this:

count = int(subprocess.check_output("grewp Ow file.txt | wc -l", shell=True))