How to check stdout in subprocess.popen in python?

Question

Python

my code:

result=subprocess.Popen(["do", 'somthing' ],stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    out=result.stdout.read().decode()
    if 'Invalid command line option' in out :
        #do something

If I use print(out), then I see that the line Invalid command line option is there, but in fact the if does not pass.
Tried several ways already, no change.