[Python-Dev] Semantics of __int__(), __index__()

Nick Coghlan ncoghlan at gmail.com
Tue Apr 2 02:44:48 CEST 2013

On Mon, Apr 1, 2013 at 12:28 AM, Mark Dickinson <dickinsm at gmail.com> wrote:

> As written, int_check would do the wrong thing for bools, too:  I
> definitely want int(True) to be 1, not True.
> For (2) and (4), it's not so clear.  Are there use-cases for an __index__
> return value that's not directly of type int?  I can't think of any offhand.

int() and operator.index() are both type coercion calls to produce true
Python integers - they will never return a subclass, and this is both
deliberate and consistent with all the other builtin types that accept an
instance of themselves as input to the constructor. Passing a subclass
instance to the base class constructor is the way you convert a subclass to
an ordinary instance of the base class:

>>> for base in (str, bytes, bytearray, int, float, complex, dict, tuple,
list, set, frozenset):
...     class subclass(base): pass
...     print("'type(base(subclass()))' is", type(base(subclass())))

'type(base(subclass()))' is <class
'type(base(subclass()))' is <class
'type(base(subclass()))' is <class
'type(base(subclass()))' is <class
'type(base(subclass()))' is <class
'type(base(subclass()))' is <class
'type(base(subclass()))' is <class 'dict'>
'type(base(subclass()))' is <class 'tuple'>
'type(base(subclass()))' is <class 'list'>
'type(base(subclass()))' is <class 'set'>
'type(base(subclass()))' is <class 'frozenset'>

There's code in the slot wrappers so that if you return a non-int object
from either __int__ or __index__, then the interpreter will complain about
it, and if you return a subclass, it will be stripped back to just the base

If the language and library reference aren't clear on this, it's a
documentation issue.


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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