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

Xavier Morel python-dev at masklinn.net
Wed Apr 3 22:01:48 CEST 2013

On 2013-04-03, at 19:46 , Barry Warsaw wrote:

> On Apr 04, 2013, at 03:04 AM, Steven D'Aprano wrote:
>> On 04/04/13 01:16, Barry Warsaw wrote:
>>> the other built-in types-as-functions, so int() calls __int__() which must
>>> return a concrete integer.
>> Why must it? I think that's the claim which must be justified, not just taken
>> as a given.  When we call n = int(something), what's the use-case for caring
>> that n is an instance of built-in int but not of a subclass, and is that
>> use-case so compelling that it must be enforced for all uses of int() etc.?
> It's a consistency-of-implementation issue.  Where built-in types are
> callable, they return concrete instances of themselves.  This is true for
> e.g. list, tuple, dict, bytes, str, and should also be true of int.

FWIW unless I missed something it's true for none of bytes, str or
float, though it's true for complex (for some reason):

    types = (int, float, complex, bytes, str)
    Obj = type('Obj', (), {
        '__{0.__name__}__'.format(t): (lambda t: lambda self:
            type('my_{0.__name__}'.format(t), (t,), {})())(t)
        for t in types

    obj = Obj()
    for t in types:
        print("{} = {} ? {}".format(t, type(t(obj)), type(t(obj)) is t))

    > python3 test.py
    <class 'int'> = <class '__main__.my_int'> ? False
    <class 'float'> = <class '__main__.my_float'> ? False
    <class 'complex'> = <class 'complex'> ? True
    <class 'bytes'> = <class '__main__.my_bytes'> ? False
    <class 'str'> = <class '__main__.my_str'> ? False

bool can not be subclassed so the question doesn't make sense for it

Broadly speaking (complex doesn't fit it), if there's a dedicated dunder
method in the data model, the only check on what it returns is that it's
a subtype of the conversion type. list, tuple and dict use non-dedicated
conversion methods (iteration or a fallback thereof) so they don't have
this occasion and have no choice but to instantiate "themselves"

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