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

Tim Delaney tim.delaney at aptare.com
Thu Apr 4 22:50:58 CEST 2013

On 5 April 2013 02:16, Ethan Furman <ethan at stoneleaf.us> wrote:

On 04/04/2013 08:01 AM, Chris Angelico wrote:

On Fri, Apr 5, 2013 at 1:59 AM, Guido van Rossum <guido at python.org> wrote:

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

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): Right, I'm aware it's possible. But who would expect it of a class? FTR I'm in the int() should return an int camp, but to answer your question: my dbf module has a Table class, but it returns either a Db3Table, FpTable, VfpTable, or ClpTable depending on arguments (if creating a new one) or the type of the table in the existing dbf file.

I fall into:

  1. int(), float(), str() etc should return that exact class (and operator.index() should return exactly an int).

  2. It could sometimes be useful for int() and index() to return a subclass of int.

So, for the int constructor, I would have the following logic (assume appropriate try/catch):

def new(cls, obj): i = obj.int()

if type(i) is int:
    return i

return i._internal_value

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