Bonsoir Eric,
Primes() is using the default _element_constructor_ from the category.
There is no check at all. Actually, it would make sense to add a
containment test at the end of the construction. Something along
def _element_constructor_(self, data):
i = super(self, X)._element_constructor_(data)
if i not in self:
raise AppropriateError
return i
Vincent
2015-02-01 18:43 UTC+01:00, Eric Gourgoulhon <egourgoul...@gmail.com>:
> Hi,
>
> While playing with prime numbers to understand facade sets in Sage, I
> noticed the following surprising behavior:
>
> sage: P = Primes() ; P
> Set of all prime numbers: 2, 3, 5, 7, ...
> sage: P.facade_for()
> (Integer Ring,)
> sage: P(4)
> 4
> sage: P(4) in P
> False
>
>
> The culprit is _element_constructor_:
>
> sage: P._element_constructor_(4)
> 4
>
>
> Actually, for any integer n, P(n) returns n. On the contratry the behavior
> of PrimeNumbers_Facade (used by Sets().example("facade") is more
> satisfactory:
>
> sage: PP = Sets().example("facade") ; PP
> Set of prime numbers (facade implementation)
> sage: PP.facade_for()
> (Integer Ring,)
> sage: PP(4)
> Traceback (most recent call last):
> ...
> ValueError: 4 is not a prime number
> sage: 4 in PP
> False
>
> Is there any reason for the above behavior of Primes() ? (Maybe I am
> missing something...)
>
> Eric.
>
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