[Python-Dev] Trashing recursive objects comparison?

Armin Rigo arigo at tunes.org
Fri Oct 17 08:54:29 EDT 2003

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

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.


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