While Python prides itsself of being a simple, straightforward programming language and being explicit is pointed out as a core value, of course, one can always discover interpreter specifics and implementation detail, that one did not expect to find when working at the surface. These days I learned more about a peculiar property of the Python garbage collector, that I would like to share.

Let’s start by introducing the problem quickly. Python manages its objects primarily by reference counting. I.e. each object stores how many times it is referenced from other places, and this reference count is updated over the runtime of the program. If the reference count drops to zero, the object cannot be reached by the Python code anymore, and the memory can be freed/reused by the interpreter.

An optional method __del__ is called by the Python interpreter when the object is about to be destroyed. This allows us to do some cleanup, for example closing database connections, etc. Typically __del__ rarely has to be defined. For our example we will use it to illustrate when the disposal of an object happens:

The situation in memory resembles this schematic:

┌────┐
│ a  │────────────┐
└────┘            ▼
┌────┐    ┌───────────────┐
│ b  │───▶│A() refcount=3 │
└────┘    └───────────────┘
┌────┐            ▲
│ c  │────────────┘
└────┘


Now we let the variables a, b, and c point to None instead of the instance A():

Changing the situation to:

┌────┐    ┌────┐
│ a  │─┬─▶│None│
└────┘ │  └────┘
┌────┐ │  ┌───────────────┐
│ b  │─┤  │A() refcount=0 │
└────┘ │  └───────────────┘
┌────┐ │
│ c  │─┘
└────┘


After we have overwritten the last reference (c) to our instance of A, the object is destroyed, which triggers a call to __del__ just before really destroying the object.

# Cyclic References

However, there are instances where the reference count cannot simply go down to zero, it is the case of cylic references:

	  ┌────┐
│ a  │
└────┘
│
▼
┌───────────────┐
┌──│A() refcount=2 │◀─┐
│  └───────────────┘  │
│  ┌───────────────┐  │
└─▶│B() refcount=1 │──┘
└───────────────┘


Setting a to None, we will still have refcounts of >= 1. For these cases, Python employs a garbage collector, some code that traverses memory and applies more complicated heuristics to discover unused objects. We can use the gc module to manually trigger a garbage collection run.

However, since A implements __del__, Python refuses to clean them, arguing that it cannot not tell, which __del__ method to call first. Instead of doing the wrong thing (invoking them in the wrong sequence), Python decides to rather do nothing – avoiding undefined behaviour, but introducing a potential memory leak.

In fact, Python will not clean any objects in the cycle, which can possibly render a huger group of objects to pollute memory (see https://docs.python.org/2/library/gc.html#gc.garbage ). We can inspect the list of objects, which could not be garbage collected:

Finally, if you remove the __del__ method from the class, you would not find these objects in gc.garbage, as Python would just dispose of them.

# Python 3

As it turns out, from Python 3.4 on, the issue I wrote about does not exist anymore. __del__ s do not impede garbage collection any more, so gc.garbage will only be filled for other reasons. For details, you can read PEP 442 and the Python docs.

Considering the adoption of Python 3.4, most Python code bases have to be careful about when to use __del__.