[Python-Dev] Semantics of int(), index() (original) (raw)

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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