Python garbage collection

Question

I have created some python code which creates an object in a loop, and in every iteration overwrites this object with a new one of the same type. This is done 10.000 times,

Answer 1

If you're creating circular references, your objects won't be deallocated immediately, but have to wait for a GC cycle to run.

You could use the weakref module to address this problem, or explicitly del your objects after use.

Answer 2

weakref can be used for circular object structured code as in the explained example

Answer 3

I think this is circular reference (though the question isn't explicit about this information.)

One way to solve this problem is to manually invoke garbage collection. When you manually run garbage collector, it will sweep circular referenced objects too.

import gc

for i in xrange(10000):
    j = myObj()
    processObj(j)
    #assuming count reference is not zero but still
    #object won't remain usable after the iteration

    if !(i%100):
        gc.collect()

Here don't run garbage collector too often because it has its own overhead, e.g. if you run garbage collector in every loop, interpretation will become extremely slow.

Answer 4

Here's one thing you can do at the REPL to force a dereferencing of a variable:

>>> x = 5
>>> x
5
>>> del x
>>> x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined

Answer 5

I found that in my case (with Python 2.5.1), with circular references involving classes that have __del__() methods, not only was garbage collection not happening in a timely manner, the __del__() methods of my objects were never getting called, even when the script exited. So I used weakref to break the circular references and all was well.

Kudos to Miles who provided all the information in his comments for me to put this together.

Answer 6

You haven't provided enough information - this depends on the specifics of the object you are creating and what else you're doing with it in the loop. If the object does not create circular references, it should be deallocated on the next iteration. For example, the code

for x in range(100000):
  obj = " " * 10000000

will not result in ever-increasing memory allocation.