Inner workings of this Python feature: Can a Python data structure reference itself?

[vasudevram]

On Sunday, May 3, 2015 at 6:30:16 PM UTC+5:30, vasudevram wrote:
> On Sunday, May 3, 2015 at 4:48:11 AM UTC+5:30, Terry Reedy wrote:
> > On 5/2/2015 4:02 PM, vasudevram wrote:
> > > Hi group,
> > >
> > > Please refer to this blog post about code showing that a Python data
> > > structure can be self-referential:
> > >
> > > http://jugad2.blogspot.in/2015/05/can-python-data-structure-reference.html
> > >
> > >  Gotten a couple of comments on it already, but interested in hearing
> > > thoughts of Python core dev team members or others who can comment on
> > > the internals of how this feature operates, why, etc.
> > 
> > Please correct the following:
> >    "g (a list) contains itself as a list item (of g)."
> > g is a dict, as you yourself later said.
> > 
> > "Case 2) But if the evaluation works in a different order, i.e. the 
> > globals() function is first called (before the variable g is created), 
> > then at this point its return value (the dict) should not contain the 
> > item with key 'g' (and value g), and it is this dict that should get 
> > assigned to the variable g. Hence when we print g, we should not see g 
> > again within it."
> > 
> > This seems like you are presenting this as a statement of fact, but you 
> > then admit it is false.  The lead in sentence should more carefully 
> > state that what follows are possible hypotheses.  one is true and the 
> > other (mostly) not.
> > 
> > The key point is the meaning of "the globals() function returns a dict 
> > representing the current global symbol table,"  "Global symbol table" is 
> > an abstraction.  In CPython, the implementation is a dict and globals 
> > returns that dict, not a copy.  Python generally does not copy objects 
> > unless requested.
> > 
> > Similarly, locals() returns a dict representing the current local symbol 
> > table. In a CPython class statement, the local symbol table is 
> > implemented with a dict, and locals() is that dict.  In a CPython def 
> > statement, the local symbol table is implemented as a C array (of 
> > pointers to PyObjects). Locals() is a dict (created just once) updated 
> > from local names in the code object and the objects in the array *at the 
> > time of the call*
> > 
> >  >>> def f(a):
> > 	g = locals()
> > 	print(id(g), g)
> > 	g = locals()
> > 	print(id(g), g)
> > 	
> >  >>> f(3)
> > 56288136 {'a': 3}
> > 56288136 {'a': 3, 'g': {...}}
> > 
> > 'Case 2" applies for the first locals() call, but only for the first.
> > 
> > I believe that there was a time when printing a recursive structure hit 
> > the recursion limit like your flatten did. But I will not reload 1.5 to 
> > check.
> > 
> > -- 
> > Terry Jan Reedy
> 
> Terry Reedy:
> 
> Thanks for the detailed answer. I have corrected the list vs. dict mistake in a comment to my original post on my blog. Don't want to edit the post itself since some readers will get it twice via feed readers.
> 
> Re. statement of fact vs. hypotheses. While I'm not sure of your exact meaning in that paragraph, I understand the concept, and yes, I was not clear enough in phrasing that part. It should have read like something along these lines:
> 
> Observations -> One or more hypotheses -> deductions -> one or more alternative conclusions.
> 
> I mixed that up a bit.
> 
> Thanks.
> - Vasudev

I may have needed to put the "experiments" step in there as well :)

See:

http://en.wikipedia.org/wiki/Scientific_method