Why does the rjust method leave a string unchanged?

I have 2 codes snippets in Python 3:

print(b'3'.rjust(4, b'0'))

b'0003'

length = str(len("str")).encode() # length as bytes
length.rjust(4, b'0') # trying to pad length
print(length)

b'3'

As I understand it, both should produce the same result as the first snippet. Why does the second snippet not work as expected?

>Solution :

length.rjust returns a new string. Strings in Python cannot be modified in place. Try:

length = length.rjust(4, b'0')