Using Enum to illustrate:

    --> class Grade(enum.Enum):
    ...   A = 4
    ...   B = 3
    ...   C = 2
    ...   D = 1
    ...   F = 0
    ...   def __index__(self):
    ...     return self._value_

    --> ['miserable'][Grade.F]
    'miserable'

    --> '%x' % Grade.F
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: %x format: a number is required, not Grade

    --> hex(Grade.F)
    '0x0'

I suggest that hex() and oct() have the same check that %x and %o do so that non-numbers are not representable as hex and octal.  

While we're at it, we should do the same for bin().  Are there any others?

I'll create a patch once we have a decision on which way to solve this issue.