python - Can't concat bytes to str

Question

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

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

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

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

The output file looks like:

test string

and then I get this error:

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

Answer 1

subprocess.check_output() returns a bytestring.

In Python 3, there's no implicit conversion between unicode (str) objects and bytes objects. If you know the encoding of the output, you can .decode() it to get a string, or you can turn the you want to add to bytes with " ".encode('ascii')