[python] Can't concat bytes to str

This is proving to be a rough transition over to python. What is going on here?:

f = open( 'myfile', 'a+' )
f.write('test string' + '\n')

key = "pass:hello"
plaintext = subprocess.check_output(['openssl', 'aes-128-cbc', '-d', '-in', test, '-base64', '-pass', key])
print (plaintext)

f.write (plaintext + '\n')
f.close()

The output file looks like:

test string

and then I get this error:

b'decryption successful\n'
Traceback (most recent call last):
  File ".../Project.py", line 36, in <module>
    f.write (plaintext + '\n')
TypeError: can't concat bytes to str

You can convert type of plaintext to string:

f.write(str(plaintext) + '\n')

f.write(plaintext)
f.write("\n".encode("utf-8"))

subprocess.check_output() returns bytes.

so you need to convert '\n' to bytes as well:

 f.write (plaintext + b'\n')

hope this helps