[Python-Dev] int() and math.trunc don't accept objects that only define __index__

[Serhiy Storchaka]

18.02.19 18:16, Rémi Lapeyre пише:
> The documentation mentions at 
> https://docs.python.org/3/reference/datamodel.html#object.__index__
> the need to always define both __index__ and __int__:
> 
>      Note: In order to have a coherent integer type class, when 
> __index__() is defined __int__() should also be defined, and both should 
> return the same value.
> 
> Nick Coghlan proposes to make __int__ defaults to __index__ when only 
> the second
> is defined and asked to open a discussion on python-dev before making 
> any change
> "as the closest equivalent we have to this right now is the "negative" 
> derivation,
> where overriding __eq__ without overriding __hash__ implicitly marks the 
> derived
> class as unhashable (look for "type->tp_hash = 
> PyObject_HashNotImplemented;").".
> 
> 
> I think the change proposed makes more sense than the current behavior and
> volunteer to implement it if it is accepted.

Should we add default implementations of __float__ and __complex__ when 
either __index__ or __int__ is defined? Currently:

 >>> class A:
...     def __int__(self): return 42
...
 >>> int(A())
42
 >>> float(A())
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: float() argument must be a string or a number, not 'A'
 >>> complex(A())
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: complex() first argument must be a string or a number, not 'A'

Or just document that in order to have a coherent integer type class, 
when __index__() or __int__() are defined __float__() and __complex__() 
should also be defined, and all should return equal values.