• Use the method decode on bytes to decode to str (unicode)
  
• Use the method encode on str to encode to bytes.
  
• A str object does not have the method decode. It is already decoded.
  
• A bytes object does not have the method encode. It is already encoded.
  

• don't use upper case in variable names, try not to use names like Str
  
• post your code in code tags (BB-Code in the forum)
  

>>> s = b'hello '
>>> w = 'world'
>>> s + w
Traceback (most recent call last):
  File "<string>", line 428, in runcode
  File "<interactive input>", line 1, in <module>
TypeError: can't concat str to bytes

# Decode from bytes to string
>>> s.decode() + w
'hello world'

>>> # The same as
>>> s.decode('utf-8') + w
'hello world

# For last example
>>> japanese = "桜の花びらたち"
>>> japanese
'桜の花びらたち'
>>> type(japanese)
<class 'str'>

# Write to disk
japanese = "桜の花びらたち"
with open('jap.txt', 'w', encoding='utf-8') as f_out:
    f_out.write(japanese)
 
# Read from disk
with open('jap.txt', encoding='utf-8') as f:
    print(f.read())

Output:
桜の花びらたち