subprocess.Popen() error (No such file or directory) when calling command with arguments as a string

You should pass the arguments as a list (recommended):

subprocess.Popen(["wc", "-l", "sorted_list.dat"], stdout=subprocess.PIPE)

Otherwise, you need to pass shell=True if you want to use the whole "wc -l sorted_list.dat" string as a command (not recommended, can be a security hazard).

subprocess.Popen("wc -l sorted_list.dat", shell=True, stdout=subprocess.PIPE)

Read more about shell=True security issues here.

In this case, to use the command as a whole string and avoid the security issues of shell=True you can use shlex

import subprocess
import shlex

command = "wc -l sorted_list.dat"
subprocess.Popen(shlex.split(command), stdout=subprocess.PIPE)