[ZODB-Dev] Lazy Instantiation

[Jim Fulton]

On Sat, Jan 22, 2011 at 9:09 AM, Tres Seaver <tseaver at palladion.com> wrote:

Thanks Tres.  I'll add few tiny fine points.

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> On 01/22/2011 08:54 AM, frits.swinkels wrote:
>> I am new to ZODB and to this list: greetings.
>>
>> I need to get some points clear about precisely when objects are fully
>> instantiated in memory. It may be a simple matter of pointing me at some
>> documentation; I read pretty much anything I could find but there may a
>> problem of your jargon being different from mine:)
>>
>> The main data structure of my application is a directed graph of nodes;
>> each node has at least three slots which contain other nodes (e.g.
>> parent, right child, left child; the parent always exists, the child
>> slots may be None). I only want to instantiate that part of the graph
>> that I am working on.
>>
>> Q1. I assume that when an object is instantiated, its "sub" objects will
>> be in the form of some kind of proxy (perhaps that ghost I met in the
>> ZODB How document?). Is this assumption correct?

Note that the ghost is not a proxy.  It is the real object and is
instantiated. It is in a special state that we call the "ghost" state.

> Yes.  The ghost has not had its instance dictionary populated yet.
> Attempting to access any instance attribute will "unghostify" the object.

>
>> Q2. Given an object, designated by the variable a, in which of the
>> following statements does the child become instantiated (activated?) ?
>> child = a.leftchild
>> name = child.name
>
> The second (assuming that the nodes in your data structure derive from a
> "persistent" base class, such as Persistent, or one of the BTrees),
>
>> Q3. Actually, before the child is instantiated, I somehow would like to
>> query the object a, whether there is anything in the slot, i.e. whether
>> the content of the slot is not None. Of course, this query should not
>> trigger the instantiation.

I assume you're talking about activation (instantiation isn't a good
word here) of the child.  Simply getting the child won't activate it,
but almost any operation on it will.

  a.child is None

won't cause the child to be activated, because it only use's the
child's address, but:

   a.child == None

will because equality comparison will attempt to access
methods on the child, which will cause it's activation.

Jim

-- 
Jim Fulton
http://www.linkedin.com/in/jimfulton