Accessing Objects Based On Their ID

[Tim Daneliuk]

This is probably obvious to you Python Geniuses (tm) out there but,
it is very late and I am experiencing Brain Fade:

Given the ID of an object, is there a way to access it? For example,
if we have the ID of a class instance, is there a way to invoke its
methods and attributes knowning only that ID? Similarly, if we have the
ID of a function, is there a way to call it?

This comes up because of an implementation I had in mind wherein I
would store the IDs of a notionally linked-list of objects - but without
the link - I just want to store their IDs in the desired order. But later,
when I want to actually use the objects I need a way to get from ID back
to something accessible in the namespace...

TIA,

 

[Diez B. Roggisch]

Given the ID of an object, is there a way to access it? For example,
if we have the ID of a class instance, is there a way to invoke its
methods and attributes knowning only that ID? Similarly, if we have the
ID of a function, is there a way to call it?

No.

This comes up because of an implementation I had in mind wherein I
would store the IDs of a notionally linked-list of objects - but without
the link - I just want to store their IDs in the desired order. But
later, when I want to actually use the objects I need a way to get from ID
back to something accessible in the namespace...

Why only the id? A list only stores a reference to the object anyway - no
copy of it. So you don't gain anything by using the id.

 

[Fredrik Lundh]

This is probably obvious to you Python Geniuses (tm) out there but,
it is very late and I am experiencing Brain Fade:

Given the ID of an object, is there a way to access it?

short answer: no.

longer answer: write a small C extension that casts an integer (or long integer)
argument to a PyObject, increments the refcount, and returns the object. or
use gc.get_objects() to get a list of all GC-aware objects, and see if your object
is in there. etc. all solutions are fragile, non-portable, and/or inefficient.

This comes up because of an implementation I had in mind wherein I
would store the IDs of a notionally linked-list of objects - but without
the link - I just want to store their IDs in the desired order. But later,
when I want to actually use the objects I need a way to get from ID back
to something accessible in the namespace...

sounds like "import weakref" might be what you need.

</F>

 

[Fredrik Lundh]

longer answer: write a small C extension that casts an integer (or long integer)

argument to a PyObject, increments the refcount, and returns the object.

here's a pointer to a pointer to such a function, btw:

http://mail.python.org/pipermail/python-list/1999-October/013715.html

(see other messages in that thread for more on why this is a stupid idea)

</F>

 

[Tim Daneliuk]

No.




Why only the id? A list only stores a reference to the object anyway - no
copy of it. So you don't gain anything by using the id.

Point taken... thanks.

 

[Terry Reedy]

Given the ID of an object, is there a way to access it?

You got the right short answer already.
Another long answer 'yes' is to keep a dictionary mapping ids to objects,
but as Diez pointed out, you need a real reason to do this.

In some respects, id() is like a siren (in the mythological sense) that
Python would be better off without.

Terry J. Reedy