On 01/01/2021 14:50, Jonathan Fine wrote:
...
To summarise, it's loops / cycles in the representation of the data
that's the problem. ... .
Final idea. Perhaps a tool to detect cycles in data might be a good idea.
Detecting cycles will involve a lot of bikeshedding. (Sorry, couldn't
resist.)
But you're right, that's a better word for discussing the problem.
Steven's problem data structures are cyclic graphs. I don't agree that
such structures are a sign one is doing something wrong (outside the
naive approach to printing them out).
If memory serves me right, what we're talking about is finding and
walking a "spanning tree" of the graph. There is a twist in our case
that we would like to insert a special object where a link was elided to
break the cycle: an object that maybe contains the original reference
but would reproduce as "..." in print.
Since it appears necessary to create a data structure with as many nodes
as there are in the tree, just to remember where we've been, we may as
well say that the required result takes the form of an iterable of the
nodes, which may subsequently be iterated by a consumer. Actually, I can
think of two styles of iterator: one where "..." objects are skipped by
a "processing" iterator, and one where they are delivered by a "print"
iterator. It is valid to do this with objects that can't be hashed, so
the structure is perhaps a dict keyed by id.
Would this be possible as a general capability? I think only if the
references were in the __dict__ of each instance. With a built-in, one
is at the mercy of the implementor. But we must have to do something
very similar to pickle arbitrary (potentially cyclic) structures, which
is likewise dependent on special help from the particular built-in type.
Can something be founded on |||__getstate__| ?
Happy New Year
Jeff Allen
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