(Using python 3 terminology)  str_subtype_new is the function that creates instances of any subtypes of bytes (ie is called by bytes_new if the requested type is not PyBytes_Type -- looks like this function's name comes from python 2).  Its approach is to create a bytes object using the same arguments, and then copy the resulting data into the subclass-instance's memory.  It does    tmp = bytes_new(&PyBytes_Type, args, kwds);    [error checking]    assert(PyBytes_CheckExact(tmp));The problem is that bytes_new can return a subclass of bytes, if the argument provides a __bytes__ method that returns a bytes-subtype.  For example    class MyBytes(bytes):        pass    class C(object):        def __bytes__(self):            return MyBytes(b"hello world")    MyBytes(C()) # fails the assertThis doesn't seem to cause any issues other than the failing assert in debug builds; it seems like the assert should just be relaxed from PyBytes_CheckExact to PyBytes_Check since that's enough to guarantee that the upcoming manipulation of the "tmp" variable is going to be valid.  Also, this would match how unicode_subtype_new behaves.This bug also applies to Python 2, since I think the relevant code is the same, though in that case it applies to str instead of bytes.

Thank you for your report Kevin and sorry for the delay. I confirm the bug and agree with your solution. The same bug exists for ints and floats.Here are patches that fix crashes.

New changesetd8e0b54ece62 by Serhiy Storchaka in branch '3.4':Issue#24731: Fixed crash on converting objects with special methodshttps://hg.python.org/cpython/rev/d8e0b54ece62New changeset80efe5cc8934 by Serhiy Storchaka in branch '3.5':Issue#24731: Fixed crash on converting objects with special methodshttps://hg.python.org/cpython/rev/80efe5cc8934New changeset1c4d256cc370 by Serhiy Storchaka in branch 'default':Issue#24731: Fixed crash on converting objects with special methodshttps://hg.python.org/cpython/rev/1c4d256cc370New changeset37158c067b25 by Serhiy Storchaka in branch '2.7':Issue#24731: Fixed crash on converting objects with special methodshttps://hg.python.org/cpython/rev/37158c067b25

New changeset2ea1a3bf448f by Serhiy Storchaka in branch '2.7':Fixed Py3k warnings in tests for issue#24731.https://hg.python.org/cpython/rev/2ea1a3bf448f

Awesome, thanks!