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

Guido van Rossum guido at python.org
Thu Apr 4 16:59:49 CEST 2013

On Thu, Apr 4, 2013 at 7:47 AM, Chris Angelico <rosuav at gmail.com> wrote:

On Fri, Apr 5, 2013 at 1:23 AM, Oscar Benjamin <oscar.j.benjamin at gmail.com> wrote: > The reason for calling int(obj) is to get an object that is precisely > of type int. When I call this I do not want any modified or additional > methods or data attached to the resulting object.

There's something I'm fundamentally not understanding about this debate, and that is: How is it that calling a class can logically return anything other than an instance of that class? Taking it to a user-defined type: class Foo: pass class Bar(Foo): pass Is there any argument that I can pass to Foo() to get back a Bar()? Would anyone expect there to be one? Sure, I could override new to do stupid things, but in terms of logical expectations, I'd expect that Foo(x) will return a Foo object, not a Bar object. Why should int be any different? What have I missed here?

A class can define a new method that returns a different object. E.g. (python 3):

class C: ... def new(cls): return 42 ... C() 42

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