Currently, it is hard to get back the integer value of a class inheriting from int, especially when overriding more and more methods. Consider this example:

class A(int):
    __repr__ = lambda self: "<A %d>"%self
    __add__ = lambda self, other: A(int(self) + other)

Both fail with "TypeError: can't convert A to int". No __int__ or __index__ methods are available; the best workaround I've found is replacing int(self) with +self or self * 1 – provided those are not overridden too.

Use cases: mainly enum-style objects, and everything that wraps semantics around an integer (playground examples are is_prime() and is_odd(), more realistic ones are is_successful() when wrapping response codes).

I've started looking into the code, and my impression is that the mp_obj_get_int function should have one more case. My initial approach was to use the mechanisms around mp_obj_class_lookup to find any native base object if it exists before failing; it turned out that if I went that way, I'd probably have to go all the way and implement a slot for it – not too friendly on RAM usage for a rarely used operation.

Looking for solutions in the other issues (sorry if I'm writing this a second time – I could swear I had attached a note about this to a duplicate bug report yesterday, but failed to find either my comment or the report I though I had found), #238 seems to provide a nice way: Treat int-conversion like a unary operator. (That's suspiciously close to the +a workaround).

Do you consider adding __int__ functionality alongside __bool__ and __len__ as MP_UNARY_OP_INT a viable solution? If so, I'd be happy to implement it.