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

Barry Warsaw barry at python.org
Wed Apr 3 16:16:12 CEST 2013

On Apr 03, 2013, at 09:17 PM, Nick Coghlan wrote:

>Perhaps we should start emitting a DeprecationWarning for int subclasses
>returned from __int__ and __index__ in 3.4?

I definitely agree with doing this for __int__(), since it's intimately tied
to int(), which is clearly a type conversion operation.  It's analogous to all
the other built-in types-as-functions, so int() calls __int__() which must
return a concrete integer.

__index__() is a bit trickier because it is not tied directly to type
conversion.  In this case, int subclasses could be valid, and as Hrvoje later
points out, returning int-subclasses from __index__() should still work for
all valid use cases.

(Bug 17576 would still be a bug in this scenario, since obj.__index__() still
needs to be called by operator.index() even when it's an int subclass.)

>(I like the idea of an explicit error over implicit conversion to the base
>type, so deprecation of subtypes makes sense as a way forward. We should
>check the other type coercion methods, too.)



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