CODEWITHSUNDEEP
python
Jeff
Jeff

Reputation: 978

Python: How do I keep track of whether an object is still in 'use'?

I have a text editing program that hands out cursors to other parts of the program that require it. The cursor consists of a two part list, [start, end], which needs to be updated every time text is inserted/removed (the start/end index gets moved forward or backwards).

When the cursor is no longer used, I want to stop updating it, since there are many and they are time consuming to update. By not in use, I mean that the object that requested it no longer references it - it no longer cares about it. (For example: it has a list of cursors to all search results for the word 'bob', and a new search was made for the word 'fred', so now it replaces its result list with a new list of new cursors... the old list and its cursors are no longer used.)

I can require that any object using the cursor calls a .finished() method when it no longer needs it. But it would be easier if I could detect when it is no longer being referenced by anything outside of the editor. How do I check this in python (I know the garbage cleanup maintains a list, and deletes it when no longer referenced)?

Upvotes: 2

Views: 121

Answers (2)

John La Rooy
John La Rooy

Reputation: 304315

In addition to what @nneonneo said, you should periodically scan through your list of cursor weak references and cull out the Nones otherwise you will end up with an ever growing list of Nones

Upvotes: 4

nneonneo
nneonneo

Reputation: 179482

Use a weak reference from the weakref module to hold your cursor reference.

When the referent of a weak reference no longer has any strong (normal) references to it, the weakref will resolve to None.

>>> import weakref
>>> class Cursor: pass
... 
>>> _ = None # suppress special _ variable
>>> a = Cursor()
>>> r = weakref.ref(a)
>>> print r()
<__main__.Cursor instance at 0x1004a2bd8>
>>> del a
>>> print r()
None

You can put all those weakrefs in a collection (or use WeakKeyDictionary, WeakValueDictionary, or WeakSet) to keep track of the various cursors you have to update.

Upvotes: 6

Related Questions