Special __bytes__ method is purposed for converting to bytes with bytes constructor (as well as __str__ and __float__ for str and float). But this doesn't work if the class is a subclass of str.

>>> class X:
...     def __bytes__(self):
...         return b'abc'
... 
>>> bytes(X())
b'abc'
>>> class Y(str):
...     def __bytes__(self):
...         return b'abc'
... 
>>> bytes(Y())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: string argument without an encoding

Other conversion special methods work in str subclass.

>>> class I(str):
...     def __int__(self):
...         return 42
... 
>>> int(I('35'))
42
>>> int(I('35'), 8)
29

New changeset 1e54adef4064 by Serhiy Storchaka in branch '3.5':
Issue #25766: Special method __bytes__() now works in str subclasses.
https://hg.python.org/cpython/rev/1e54adef4064

New changeset af0b95fb1c19 by Serhiy Storchaka in branch 'default':
Issue #25766: Special method __bytes__() now works in str subclasses.
https://hg.python.org/cpython/rev/af0b95fb1c19