On Jan 23, 2009, at 12:41 PM, William Stein wrote:
> On Fri, Jan 23, 2009 at 9:24 AM, Martin Albrecht
> <m...@informatik.uni-bremen.de> wrote:
>> On Friday 23 January 2009, you wrote:
>>> Hi Martin,
>>>
>>> I don't like the hash doctest changes you introduced in #4965.
>>> They should be
>>>
>>> sage: hash(GF(3^4),'a')
>>>
>>> or
>>>
>>> sage: h = hash(GF(3^4),'a')
>>>
>>> instead of
>>>
>>> sage: {GF(3^4, 'a'):1} #indirect doctest
>>> {Finite Field in a of size 3^4: 1}
>>
>> I disagree. h = hash(GF(3^4,'a')) encodes no information
>> whatsoever except
>> that the function actually returns something. Printing the
>> explicit values is
>> weird also because it just doesn't matter what that value is if it is
>> sufficiently pseudo-random.
>>
>> sage: {GF(3^4, 'a'):1} #indirect doctest
>> {Finite Field in a of size 3^4: 1}
>>
>> tests one very important use-case.
>>
>> Martin
>
> I disagree. {GF(3^4, 'a'):1} also tests nothing whatsover except
> that the
> hash function actually returns something. Using a dictionary with
> key 1
> is weird also because it just doens't matter what hash was computed.
>
> I think we are both wrong. Doctesting of __hash__ should be done by
> a function:
>
> sage: test.hash(GF(3^4, 'a'))
> 'ok' # or not raise exception
>
> The advantages of doing this:
>
> (1) It is readable -- it's clear what is being done -- a function
> made for testing hashing is read.
>
> (2) it centralizes the design decision about what the actual test
> should be. One huge objection to you changing all the doctests in
> your files, is that they are now inconsistent with the rest of sage,
> which is very jarring for readers (it certainly was for me).
>
> (3) it enables all kinds of interestings tests to be put in
> test.hash. For example, suppose I wonder if there are any objects in
> Sage with the property that calling hash multiple times doesn't get
> "very fast" after the first call (e.g, the bug in your
> finite_field_givaro.pyx hash that I fixed). I can easily just change
> test.hash to test for that, run the test suite, and it will tell me
> everywhere this happens. Or suppose I want to hunt for instances
> where hashing is inconsistent (doesn't always return the same thing on
> 10 calls say) -- again that's easy. Suppose we decide that
> set([GF(3^4,'a'))]) is a good test -- we could trivially add that.
This sounds like an interesting proposal, but where does test come
from? I guess it's some global variable of some sort.
I don't like how it obscures what is actually being tested, i.e. the
desirable properties and design decisions of the hash. nor are there
ways to overwrite it (e.g. one might want hash(5) == hash(R(5)) for
most rings R, but if R=GF(3) this will fail.)
- Robert
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