Message 265196 - Python tracker

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.