Hashable object with self references OR how to create a tuple that refers to itself

[Prasad, Ramit]

> > I am trying to create a collection of hashable objects, where each
> > object contains references to
> > other objects in the collection.  The references may be circular.
> >
> > To simplify, one can define
> >     x= list()
> >     x.append(x)
> > which satisfies x == [x].
> > Can I create a similar object for tuples which satisfies x == (x,)?
> 
> You can create a tuple in "C" and then put a reference to itself into it,
> but I am quite convinced that you cannot do it in Python itself.
> (Of course, you could use "cython" to generate C code with a source
> language
> very similar to Python).
> 
> But, you do not need tuples; You could use a standard class:
> 
> >>> class C(object): pass
> ...
> >>> c=C()
> >>> c.c=c
> >>> d=dict(c=c)
> >>> d
> {'c': <__main__.C object at 0xb737f86c>}
>

Using a class is a good approach. You can also override __contains__ 
for the custom classes internal collection so instead of x==[x,] you 
would use x in obj where obj is a collection with the equivalent [x,].

Not entirely sure why Dieter is bringing up C code / Cython... 

Just as a note, if you store references to an object A in the collection
in another object B in the collection and then try to remove A from the
collection it will not get garbage collected nor removed from B. To
allow for garbage collection you should store a weakref instead.
http://docs.python.org/library/weakref.html 

Ramit


Ramit Prasad | JPMorgan Chase Investment Bank | Currencies Technology
712 Main Street | Houston, TX 77002
work phone: 713 - 216 - 5423

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