[New-bugs-announce] [issue26984] int() can return not exact int instance

[Serhiy Storchaka]

New submission from Serhiy Storchaka:

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.

----------
components: Interpreter Core
messages: 265196
nosy: mark.dickinson, serhiy.storchaka
priority: normal
severity: normal
status: open
title: int() can return not exact int instance
type: behavior
versions: Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue26984>
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