Check if a unix command returns nothing

I am trying to see if myfile returns nothing after a grep from within python, something like this:

command="cat my_file.txt | grep 'string_to_find'"

if os.system(command) == '':
   print('nothing found')

>Solution :

Converting comment to answer:

command="cat my_file.txt | grep 'string_to_find'"

if os.popen(command).read() == '':
   print('nothing found')