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 changeset1e54adef4064 by Serhiy Storchaka in branch '3.5':Issue#25766: Special method __bytes__() now works in str subclasses.https://hg.python.org/cpython/rev/1e54adef4064New changesetaf0b95fb1c19 by Serhiy Storchaka in branch 'default':Issue#25766: Special method __bytes__() now works in str subclasses.https://hg.python.org/cpython/rev/af0b95fb1c19