Extracting elements over multiple lists?

[Steven D'Aprano]

On Tue, 15 Nov 2011 17:01:23 +0000, Prasad, Ramit wrote:

> Can you expand on why 'del' is "tricky"/misleading?

People often imagine that the del statement sends a message to the object 
"please delete yourself", which then calls the __del__ method. That is 
incorrect.

"del x" is an unbinding operation, it removes the *name* "x" from the 
current namespace. As a side-effect, if the object which was bound to x 
no longer has any other references to it, then the garbage collector will 
delete it and __del__ may be called.

(I say "may be called" rather than "will" because there are circumstances 
where __del__ methods won't get called, such as during interpreter 
shutdown.)

On the other hand, "del x[i]" does work like the way people expect. It 
deletes items from collections (lists, dicts, etc.) and does so by 
calling the method x.__delitem__(i). This also may cause the garbage 
collector to delete the object which was at x[i] if that was the last 
reference to that object.

CPython's implementation keeps a count of references for each object, and 
the garbage collector deletes the object immediately that reference count 
reaches zero. This is fast, simple, deterministic (objects will be 
predictably deleted as soon as they can be), but simple-minded, and so it 
is aided by a second garbage collector which runs periodically, looking 
for reference cycles. You can set how often this second garbage collector 
runs using the gc module.

Jython uses the Java garbage collector, and IronPython the .Net garbage 
collector. Neither are reference counters, and (as far as I know) neither 
guarantees that objects will be deleted as soon as they are free to be 
deleted. They will be deleted whenever the garbage collector gets around 
to it.


-- 
Steven