The int constructor can return an instance of int subclass. >>> class BadTrunc: ... def __trunc__(self): ... return True ... >>> int(BadTrunc()) True When __int__ returns non-exact int, at least a warning is emitted: >>> class BadInt: ... def __int__(self): ... return True ... >>> int(BadInt()) __main__:1: DeprecationWarning: __int__ returned non-int (type bool). The ability to return an instance of a strict subclass of int is deprecated, and may be removed in a future version of Python. True The constructor of int subclass always return an instance of correct type: >>> class IntSubclass(int): ... pass ... >>> type(IntSubclass(BadTrunc())) <class '__main__.IntSubclass'> >>> type(IntSubclass(BadInt())) __main__:1: DeprecationWarning: __int__ returned non-int (type bool). The ability to return an instance of a strict subclass of int is deprecated, and may be removed in a future version of Python. <class '__main__.IntSubclass'> I don't know if it is worth to deprecate __trunc__ returning non-exact int, since this special method is used in math.trunc(). But I think that the int constructor should convert its result to exact int. If some preparatory period is needed, it can first start to emit FutureWarning.
The patch LGTM. I'm still in two minds about whether `__trunc__` should be required to return a strict `int`, but for now it's probably better to go with the status quo.
