hi Jason,
> I was a bit surprised to find out just how lazy LazyFamily is:
>
> sage: f = Family(Zmod(3), lambda i: 2*i) # This will be lazy by default
> sage: f[2] # I think this should be 1
> 4
> sage: f['spam'] # I think this should fail
> 'spamspam'
>
> Compare with:
> sage: f = Family([i for i in Zmod(3)], lambda i: 2*i)
> sage: f[2]
> 1
> sage: f['spam']
> KeyError: 'spam'
>
> I expected that the __getitem__ method for lazy family would
>
> 1) Check that the value given is (or can be coerced to) a key, and
> 2) If a coercion is necessary, call the function on the coerced item.
>
> But I guess that not everyone agrees since one of the tests in LazyFamily is
>
> sage: f = LazyFamily([3,4,7], lambda i: 2*i)
> sage: f[5]
> 10
>
> Perhaps there are arguments about pythonic code and the value of
> duck-typing that I'm not appreciating here. Or maybe the issue is
> performance. But can someone say a few words about the rationale for
> this decision?
When calling Family(keys, functions) keys can be pretty anything: lists,
tuple, parent in the category EnumeratedSets() or probably anythng which is
iterable... We didn't assume that x in keys or keys(x) has any meaning. For
example, I think the following is legal:
sage: class toto(object):
....: def __iter__(self):
....: for i in [1,2,3]: yield i
....:
sage: bla = toto()
sage: Family(bla, lambda x: x^2)
Lazy family (<lambda>(i))_{i in <__main__.toto object at 0x45f7f10>}
Maybe this is too much permissive. So you are suggesting we should also
require that keys should also implement __contains__. In what concerns
coercion, please remember that a common usage is to lists as keys...
> On a related note, the following *really* looks like a bug to me:
>
> sage: f = Family(Zmod(3), lambda i: 2*i, lazy=False)
> sage: f
> Lazy family (<lambda>(i))_{i in Ring of integers modulo 3}
>
> Should we really just silently ignore the intent here, or should
>
> Family(S, f, lazy=False) always return Family([i for i in S], f)
>
> (I guess the default for lazy should then be made 'None' so that 'True',
> 'False' and 'None' could all have different behaviors.)
This is clearly a bug ! Thanks for pointing this out.
Cheers,
Florent
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