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.