#25586 closed enhancement (fixed)
py3: adding hash function for orders and fraction fields
Reported by:  chapoton  Owned by:  

Priority:  major  Milestone:  sage8.3 
Component:  python3  Keywords:  days94 
Cc:  tscrim, embray, jdemeyer  Merged in:  
Authors:  Frédéric Chapoton  Reviewers:  Vincent Delecroix, Travis Scrimshaw, Erik Bray 
Report Upstream:  N/A  Work issues:  
Branch:  6806c37 (Commits)  Commit:  
Dependencies:  Stopgaps: 
Description (last modified by )
+ a few other details
Change History (39)
comment:1 Changed 22 months ago by
 Branch set to u/chapoton/25586
 Commit set to a0f9b866f19dd5e3843cb4e11aac8e3a23be067e
 Status changed from new to needs_review
comment:2 Changed 22 months ago by
 Description modified (diff)
 Summary changed from py3: adding some hash function to py3: adding hash function for congruence groups and fraction fields
comment:3 Changed 22 months ago by
 Status changed from needs_review to needs_work
some failing doctests
comment:4 Changed 22 months ago by
 Commit changed from a0f9b866f19dd5e3843cb4e11aac8e3a23be067e to ac1563141fc541500802fefa24c07cfdd7c85557
Branch pushed to git repo; I updated commit sha1. New commits:
ac15631  trac 25586 fixing new hash of congruence groups

comment:5 Changed 22 months ago by
 Status changed from needs_work to needs_review
comment:6 Changed 22 months ago by
Similar to #25591: the hash of a fraction field should be different from the hash of its underlying ring.
What you did for congruence subgroups is not a very good idea as it will break equality
sage: G = Gamma(2) sage: H = G.as_permutation_group() sage: G == H True
One (expensive) possibility for now would be
def __hash__(self): return hash(self.as_permutation_group())
comment:7 Changed 22 months ago by
 Reviewers set to Vincent Delecroix
 Status changed from needs_review to needs_work
comment:8 Changed 22 months ago by
 Commit changed from ac1563141fc541500802fefa24c07cfdd7c85557 to a56f9735ea74cacadf2947165fb7a191f8345fca
Branch pushed to git repo; I updated commit sha1. New commits:
a56f973  fixing hash of fraction fields and congruence groups

comment:9 Changed 22 months ago by
 Status changed from needs_work to needs_review
Merci. C'est fait aussi.
comment:10 Changed 22 months ago by
ping ?
comment:11 Changed 22 months ago by
 Status changed from needs_review to positive_review
désolé... d'autres sollicitations.
comment:12 Changed 22 months ago by
Je comprends. Merci beaucoup.
comment:14 Changed 21 months ago by
 Commit changed from a56f9735ea74cacadf2947165fb7a191f8345fca to 97d8e09ab04a0c8551d7ba64540d910a1287ae60
Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:
97d8e09  py3: adding hash to orders and fraction fields + other details

comment:15 Changed 21 months ago by
 Status changed from needs_work to needs_review
 Summary changed from py3: adding hash function for congruence groups and fraction fields to py3: adding hash function for orders and fraction fields
the case of congruence groups being harder, it is no longer handled here.
comment:16 Changed 21 months ago by
ping ? I have removed the controversial part..
EDIT: and this is an important ticket for progress in python3
comment:17 Changed 21 months ago by
and bot is green
comment:19 Changed 21 months ago by
 Reviewers changed from Vincent Delecroix to Vincent Delecroix, Travis Scrimshaw
 Status changed from needs_review to positive_review
LGTM.
comment:20 Changed 21 months ago by
 Status changed from positive_review to needs_work
More list(map(...))
s that might as well be list comprehensions:

src/sage/manifolds/differentiable/affine_connection.py
diff git a/src/sage/manifolds/differentiable/affine_connection.py b/src/sage/manifolds/differentiable/affine_connection.py index e1d6601..e307a9c 100644
a b class AffineConnection(SageObject): 1186 1186 if index_labels is None and isinstance(frame, CoordFrame) and \ 1187 1187 coordinate_labels: 1188 1188 ch = frame.chart() 1189 index_labels = map(str, ch[:])1190 index_latex_labels = map(latex, ch[:])1189 index_labels = list(map(str, ch[:])) 1190 index_latex_labels = list(map(latex, ch[:])) 1191 1191 return self.coef(frame=frame).display(symbol, 1192 1192 latex_symbol=latex_symbol, index_positions='udd', 1193 1193 index_labels=index_labels, index_latex_labels=index_latex_labels,
I think I've mentioned elsewhere (and forgive me if this concern was dismissed and I didn't see it), but I don't like these usesless hash example tests like:

src/sage/rings/fraction_field.py
diff git a/src/sage/rings/fraction_field.py b/src/sage/rings/fraction_field.py index 201e132..024d83d 100644
a b class FractionField_generic(ring.Field): 688 688 """ 689 689 return not (self == other) 690 690 691 def __hash__(self): 692 """ 693 Compute the hash of ``self``. 694 695 EXAMPLES:: 696 697 sage: h = hash(Frac(ZZ['x'])) 698 """ 699 return hash(self._R) ^ 147068341996611 700 691 701 def ngens(self): 692 702 """ 693 703 This is the same as for the parent object.
All this tests is that, at best, it doesn't crash (which is almost completely trivial). It should instead demonstrate at least a case where two hashes compare the same as expected, and where two hashes differ, as expected.
I agree it's annoying to write these tests which is why I've been pokey about making tickets for __hash__
implementations in the first place :)
comment:21 Changed 21 months ago by
 Commit changed from 97d8e09ab04a0c8551d7ba64540d910a1287ae60 to dab8b92a5d70ce10554b9faafcc0cc5ca2c3d929
comment:23 followup: ↓ 33 Changed 21 months ago by
This is a stupid question and I think I might have already answered it in my head but, just for my edification:

src/sage/rings/fraction_field.py
diff git a/src/sage/rings/fraction_field.py b/src/sage/rings/fraction_field.py index 201e132..909f10b 100644
a b class FractionField_generic(ring.Field): 688 688 """ 689 689 return not (self == other) 690 690 691 def __hash__(self): 692 """ 693 Compute the hash of ``self``. 694 695 EXAMPLES:: 696 697 sage: h0 = hash(Frac(ZZ['x'])) 698 sage: h1 = hash(Frac(ZZ['x'])) 699 sage: h2 = hash(Frac(QQ['x'])) 700 sage: h3 = hash(ZZ['x']) 701 sage: h0 == h1 and h1 != h2 and h1 != h3 702 True 703 """ 704 return hash(self._R) ^ 147068341996611 705 691 706 def ngens(self): 692 707 """ 693 708 This is the same as for the parent object.
Why not just hash(self._R)
? Is it possible for instances of these fields to have a parent ring that's equivalent to itself, or something (in which case we'd need this in order for them to have unique hashes)? And where does the magic number come from?
comment:24 Changed 21 months ago by
This was modeled after the case of Laurent Polynomial at the request of Vincent, to avoid having trivially the same hash. The number is a random number.
comment:25 Changed 21 months ago by
I see, he wrote:
For Laurent polynomial ring it is not a good idea to give it the same hash as the underlying multipolynomial ring. You would better xor it with a random number
But why is that? It doesn't matter if they have the same hash unless they can also compare equal to each other.
In any case, even if there is a good reason, this should probably be noted. It's not obvious.
comment:26 Changed 21 months ago by
My guess is that maybe you can have a polynomial ring over elements of another polynomial ring, so in that case it does make sense to do that.
comment:27 Changed 21 months ago by
Do you want a note in the doc of the hash method ?
comment:28 Changed 21 months ago by
In the doc or just in a comment. Up to you.
comment:29 Changed 21 months ago by
 Commit changed from dab8b92a5d70ce10554b9faafcc0cc5ca2c3d929 to 6806c3791dcd65a2648e421a26f506616b111403
Branch pushed to git repo; I updated commit sha1. New commits:
6806c37  trac 25586 adding comment

comment:30 Changed 21 months ago by
done
comment:31 Changed 21 months ago by
ping ?
comment:32 followup: ↓ 39 Changed 21 months ago by
PING ? This is a major hurting point..
comment:33 in reply to: ↑ 23 Changed 21 months ago by
Replying to embray:
This is a stupid question and I think I might have already answered it in my head but, just for my edification:
src/sage/rings/fraction_field.py
diff git a/src/sage/rings/fraction_field.py b/src/sage/rings/fraction_field.py index 201e132..909f10b 100644
a b class FractionField_generic(ring.Field): 688 688 """ 689 689 return not (self == other) 690 690 691 def __hash__(self): 692 """ 693 Compute the hash of ``self``. 694 695 EXAMPLES:: 696 697 sage: h0 = hash(Frac(ZZ['x'])) 698 sage: h1 = hash(Frac(ZZ['x'])) 699 sage: h2 = hash(Frac(QQ['x'])) 700 sage: h3 = hash(ZZ['x']) 701 sage: h0 == h1 and h1 != h2 and h1 != h3 702 True 703 """ 704 return hash(self._R) ^ 147068341996611 705 691 706 def ngens(self): 692 707 """ 693 708 This is the same as for the parent object. Why not just
hash(self._R)
?
To avoid collisions. We do have dictionaries whose keys are parent (e.g. coercions).
comment:34 Changed 21 months ago by
For matter of equality, it is true that the polynomial ring on the zero ring is isomorphic to the base ring. Though, Sage does not know about it.
sage: S = Zmod(1) sage: R = S['x'] sage: S == R False
And even worse
sage: f = R.coerce_map_from(S) sage: f.is_injective() True sage: f.is_surjective() False
comment:35 followup: ↓ 36 Changed 21 months ago by
comment:36 in reply to: ↑ 35 ; followup: ↓ 37 Changed 21 months ago by
 Keywords days94 added
 Reviewers changed from Vincent Delecroix, Travis Scrimshaw to Vincent Delecroix, Travis Scrimshaw, Erik Bray
 Status changed from needs_review to positive_review
Replying to vdelecroix:
(see #25586 for the bug about surjectivity)
See this ticket?
I am going to set this back to positive review as Erik's concerns have been addressed.
comment:37 in reply to: ↑ 36 Changed 21 months ago by
Replying to tscrim:
Replying to vdelecroix:
(see #25586 for the bug about surjectivity)
See this ticket?
Oups: #25753
I am going to set this back to positive review as Erik's concerns have been addressed.
comment:38 Changed 21 months ago by
 Branch changed from u/chapoton/25586 to 6806c3791dcd65a2648e421a26f506616b111403
 Resolution set to fixed
 Status changed from positive_review to closed
comment:39 in reply to: ↑ 32 Changed 21 months ago by
 Commit 6806c3791dcd65a2648e421a26f506616b111403 deleted
Replying to chapoton:
PING ? This is a major hurting point..
Please try to show some patience. I have tickets that I believe are important that haven't been merged or even reviewed in months (a situation I'm not happy with, but there is only so much attention to go around).
If you've already fixed the issue then there's nothing holding you backthis is why I have my private Python 3 brancheverything I fix gets merged into it immediately and then I can carry on (even if that fix is not ultimately the accepted fix; at least it works in the meantime).
The rest of us have vacations, family, sickness, and other tasks to work on and adding "ping" to a ticket does not (at least in my case) make it any more likely that I'll see it sooner.
New commits:
py3: adding hash to congruence groups and fraction fields + other details