Hi

Rather, it's that the data has a loop (or cycle) in it. A simple example of this is

    >>> x = []
    >>> x.append(x)
    >>> x
    [[...]]
    >>> x[0] is x
    True

So far as I know, it's not possible to create such using only immutable objects. And anything created using only immutable objects will have a hash.

A beginner can easily by mistake create an object such as x above, and without the short-circuit provided by

    >>> x
    [[...]]

the result is an unprintable object, that further raises an exception (or worse). That's really bad for the poor beginner.

As a first approximation, my opinion is that data having such a loop / cycle in it is at least a code smell, and perhaps worse. However, there may be examples that I've not considered that would make me change my mind.

By the way, in the mathematics of the symmetric group (permutations) the two tuples

    >>> (0, 1, 2, 3)

are different representations of the same cycle (where here the word cycle has a different meaning).

Also by the way, determining if two vertex-edge graphs are isomorphic is a hard problem. I've been told that the best tool for this is

To summarise, it's loops / cycles in the representation of the data that's the problem. Without a use case to the contrary, I'd say don't do this. That's my opinion. Yours may be different.

Final idea. Perhaps a tool to detect cycles in data might be a good idea.

Finally, Happy New Year.

 -- 

Jonathan