[Patches] [ python-Patches-693221 ] fix bug 625698, speed up some comparisons

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Patches item #693221, was opened at 2003-02-25 21:57
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Category: Core (C code)
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
Submitted By: Michael Stone (mbrierst)
Assigned to: Nobody/Anonymous (nobody)
Summary: fix bug 625698, speed up some comparisons

Initial Comment:
This patch, patchrecursion GREATLY speeds up
comparisons of recursive objects, fixing bug 625698.

It works by throwing a new exception,
a RecursionError, to break out of the
current deeply nested comparison, and
then redoing the comparison by the
already established technique for recursive
objects.

I believe the only costs to normal 
non-recursive comparisons is two
additional C comparisons.

Backwards compatibility problems will
occur only in code that attempts to
handle ALL exceptions while doing comparisons.
Of course, this is always a bad practice,
so hopefully not much code does this.  Also,
most code probably doesn't attempt to handle
exceptions within the actual comparison
routines, instead catching them elsewhere,
which will be fine.



Another patch is also attached, patchequal,
to handle some of the common cases where this
comes up even faster.

While it is true that python wants to have
an == operator that does not define an equality
relation, I believe that when == is not an
equality relation python has few obligations
to be nice to it.  In particular, suppose we have
an object NaN != NaN.  I would say that
(NaN,) == (NaN,)
[NaN] == [NaN]
{NaN:1} == {NaN:1}
all intuitively still seem true.  Isn't it obvious
that if two lists contain the SAME objects, they
are equal?  That is, even though NaN does not
equal itself, lists, tuples, and dicts have no
obligation to respect the underlying == operator.  
I don't see any documentation saying
they will respect it (though maybe I'm just missing it).
patchequal assumes, ONLY for purposes of list tuple
dict comparisons and dict lookups, that if id(a) == id(b),
then a == b.  Note there is some precedence for this
patch,
d = {NaN:1}  NaN in d  => True
cmp(NaN, NaN) is 0 (though I suspect this is a mistake).

This will cause backwards compatibility problems
only for people using non-equality relation =='s,
and probably not even for them, as they already
probably use a trick to emulate this patch when
they compare lists and such containing these
weird objects.


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