#19331: Make Element.__hash__ return 0 by default
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Reporter: vdelecroix | Owner:
Type: enhancement | Status: needs_review
Priority: blocker | Milestone: sage-6.9
Component: misc | Resolution:
Keywords: | Merged in:
Authors: Vincent Delecroix | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/vdelecroix/19331 | 64053a2645456e605061a24952568e3c5e1f51e2
Dependencies: | Stopgaps:
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Comment (by nbruin):
Changing the runtime cost of data structures from what it should be should
not be taken lightly. I don't think this change leads to acceptable
behaviour (and it will be harder to find what's wrong: a hashing error is
easier to debug)
{{{
class A(object):
def __init__(self,N):
self.N=N
def __hash__(self):
return 0
def __eq__(self,other):
return isinstance(other,A) and self.N==other.N
def __call__(self):
return self.N
class B(object):
def __init__(self,N):
self.N=N
def __hash__(self):
return hash(self.N)
def __eq__(self,other):
return isinstance(other,B) and self.N==other.N
def __call__(self):
return self.N
@cached_function
def T(a):
return a()
}}}
{{{
sage: %timeit( [T(B(n)) for n in range(10000)])
100 loops, best of 3: 14.3 ms per loop
sage: %timeit( [T(A(n)) for n in range(10000)])
1 loops, best of 3: 25.8 s per loop
}}}
Or, with a smaller cache it's still bad (I've cleaned the cache):
{{{
sage: %timeit( [T(A(n)) for n in range(100)])
100 loops, best of 3: 2.22 ms per loop
sage: %timeit( [T(B(n)) for n in range(100)])
10000 loops, best of 3: 181 µs per loop
}}}
Even for a dictionary of size 10, the trivial hash still causes a 4x
slowdown.
A change like this can push code that runs correctly in sage at present to
a point where running it is not practical. I seriously think there may be
a user out there that has an application that depends on this code. This
change would break sage for him in that respect. I am strongly -1 to
introducing something like that in a release candidate, just before
release, because we wouldn't catch it.
--
Ticket URL: <http://trac.sagemath.org/ticket/19331#comment:4>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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