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.

> But I think that the int constructor should convert its result to exact int. Agreed. See also issue 17576 and previous discussions on python-dev (thread started in March, but most of the messages in April): - https://mail.python.org/pipermail/python-dev/2013-March/125022.html - https://mail.python.org/pipermail/python-dev/2013-April/125042.html I don't have strong feelings about `__trunc__` (or `__floor__` and `__ceil__`); it seems fine to me for those to be returning something other than a strict int.

Proposed patch makes int() always returning exact int.

Could you please make a review Mark?

> Could you please make a review Mark? Sorry, Serhiy. I missed this. I've got some time off coming up, so I plan to look at this in the next few days.

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.

New changeset 81f229262921 by Serhiy Storchaka in branch 'default': Issue #26984: int() now always returns an instance of exact int. https://hg.python.org/cpython/rev/81f229262921