Hello all,
I'm bringing (again) the subject of comparison of recursive objects to the
table because I just happened to write a buggy piece of code:
class X:
def __eq__(self, other):
return self.content == other
This code was buggy because 'self.content' could occasionally be 'self'. In
this case it should have triggered an infinite recursion, and I should have
got a nice (if a bit long) RuntimeError traceback that told me where the
problem was. At least, this is how I would expect my piece of code to
misbehave.
Instead, the answer was 'True', whatever 'other' actually was. Puzzlement
would have gained me if I had no idea about what a bisimulation, or graph
isomorphism, is, and what Python's implementation of that idea is.
Quoting Tim on bug #625698:
> As Erik's latest example shows, the outcome isn't always
> particularly well defined either. An alternative to speeding
> this
> silliness is to raise an exception instead when recursive
> objects are detected. There was some hack value in doing
> the graph isomorphism bit, but no real practical value I can
> see.
If the pretty academic subject of graph isomorphisms is well-worn enough to be
sent to the trash, I'll submit a patch that just removes all this code and
instead use the existing sys.recursionlimit counter to catch infinite
recursions and throw the usual RuntimeError.
Armin
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