It looks it's essential to call decode() function to make use of actual string data even after calling base64.b64decode over base64 encoded string. Because never forget it always return bytes literals.
import base64
conv_bytes = bytes('your string', 'utf-8')
print(conv_bytes) # b'your string'
encoded_str = base64.b64encode(conv_bytes)
print(encoded_str) # b'eW91ciBzdHJpbmc='
print(base64.b64decode(encoded_str)) # b'your string'
print(base64.b64decode(encoded_str).decode()) # your string