Documentation of `hex()` on Python 2 says that custom objects need to implement `__index__` to support it. Based on my tests that doesn't work but `__hex__` is needed instead. Docs are at 
https://docs.python.org/2/library/functions.html?highlight=hex#hex and here's an example session:

Python 2.7.6 (default, Oct 26 2016, 20:30:19) 
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class Hex(object):
...     def __index__(self):
...         return 255
... 
>>> hex(Hex())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: hex() argument can't be converted to hex
>>> 
>>> class Hex(object):
...     def __hex__(self):
...         return hex(255)
... 
>>> hex(Hex())
'0xff'


 
Assuming this is fixed, should probably note that with Python 3 you actually *need* to implement `__index__` and `__hex__` has no effect.

Python 3 uses __index__ for bin(), oct(), and hex(), but Python 2 only uses __index__ for bin() and otherwise uses __oct__ and __hex__. Antoine overlooked this when updating the 2.7 docs for hex() in issue 16665.

Attached patch changes the py2.7 documentation to point out that the method name is __hex__ and its return type is a string.

LGTM.

New changeset d7804789368a by Mariatta Wijaya in branch '2.7':
Issue #29329: Improve documentation for hex(). Patch by Ammar Askar
https://hg.python.org/cpython/rev/d7804789368a

Thanks, Pekka, Eryk, Ammar, and Zachary :)

New changeset d03fd5d041b6f514daf0aabc0c159d8727c33980 by Mariatta Wijaya in branch '2.7':
Issue #29329: Improve documentation for hex(). Patch by Ammar Askar
https://github.com/python/cpython/commit/d03fd5d041b6f514daf0aabc0c159d8727c33980