2. Background

2.1. Understanding object release in Python

This section explains how Python releases objects, the role of the Python garbage collector, the role of object references, and how memory leaks can exist in Python. It is a rather brief description just enough to understand what is relevant to your Python program. Unless otherwise stated, the description applies to CPython with its generational cyclic garbage collector (introduced in CPython 2.0).

Python has two mechanisms for releasing objects:

  • Immediate release based on reference counting:

    If the reference count of an object drops to zero (e.g. because its referencing variable goes out of scope), the reference count of all objects it references are decreased by one, and the original object is immediately released.

    Those referenced objects whose reference count drops to zero as a result of being decreased will in turn be immediately released.

    If it triggers, this process always succeeds. If the original object is involved in circular references, the process does not trigger in the first place, because the reference count of the original object never drops to zero.

  • Asynchronous release during garbage collection:

    Objects that can possibly be involved in circular references are tracked by the Python garbage collector. Python schedules runs of the garbage collector based on object allocation and deallocation heuristics. The garbage collector is able to release isolated sets of objects with circular references by breaking up these circular references.

    As an optimization, the garbage collector has 3 generations, and the heuristics are optimized such that younger generations are collected more often than older generations.

    There are circumstances whereby multiple collection runs are necessary to release objects, and where objects can end up as uncollectable, meaning they normally stay around until the Python process terminates.

When an object is created, Python decides whether or not the object is tracked by the garbage collector. Whether an object is tracked or not can change over the lifetime of the object. The gc.is_tracked() function returns whether a particular object is currently tracked or untracked.

Untracked objects are not listed in any generation of the garbage collector, so their release can only be done by the reference counting mechanism. Python guarantees that objects that are untracked are not involved in circular object references so their reference count does have a chance to drop to zero.

Tracked objects may or may not be involved in circular references. If they are not, their release happens via the reference counting mechanism. If they are, a garbage collection run (on the generation they are in) can release them. During each collection run on a generation, the garbage collector examines the objects in that generation to find isolated sets of objects with circular references that are unreachable from outside of themselves. The garbage collector tries to break up circular references to release such sets of objects.

Objects that become tracked (either at object creation or later) are always put into the first generation of the garbage collector. When a garbage collection run on a particular generation does not succeed in releasing an unreachable isolated set of objects, the objects are moved into the next (older) generation.

Objects in the last (oldest) generation that survive a collection run stay in the last generation. They will be attempted to be released again and again during future runs of the garbage collector on the last generation. In most cases, that will not succeed, but there are cases where it will. Unreachable isolated set of objects in the last generation of the garbage collector are called uncollectable objects and unless a future run succeeds in releasing them, they remain allocated until the Python process terminates.

Uncollectable objects may be considered memory leaks, but this distinction is not black and white because they may be successfully released in a future run of the garbage collector on the last generation.

Among the possible reasons for objects to become uncollectable are:

  • Before Python 3.4, the presence of the __del__() method in a set of unreachable objects involved in circular references caused these objects to be uncollectable. The reason is that Python cannot safely decide for an order in which the __del__() methods should be invoked, because they might be using the reference to another object in the cycle. In Python 3.4, PEP 442 – Safe object finalization was implemented which ensures that the __del__() methods are invoked exactly once in all cases, but in a set of objects that has reference cycles, the order of invocation is undefined. Also, this change no longer causes objects with circular references to become uncollectable just because they have a __del__() method.
  • Reference counting bugs in Python modules implemented in native languages such as C may cause objects to be uncollectable. For example, a Python module implemented in C could properly increase an object reference upon creation but forget to decrease it upon deletion. That will prevent the reference count from dropping to zero and thus will successfully prevent not only the immediate object release but also any future attempt of the garbage collector to release it.

If you want to understand this in more detail, here are a few good resources:

2.2. The issues with collected and uncollectable objects

For short-running Python programs (e.g. command line utilities), it is mostly not so important if there are some memory leaks and other resource leaks. On most operating systems, resource cleanup at process termination is very thorough and resources such as open files are cleaned up properly. This should not be understood as advocating to be careless there, but the negative effect is less severe on short-running programs compared to long-running programs.

If your Python package provides modules for use by other code, you usually cannot predict whether it will be used in short-running or long-running programs. Therefore, resource usage in a Python module should be designed with the worst case assumption in mind, i.e. that it is used by an infinitely running piece of code.

The remainder of this section explains the issues with collected and uncollectable objects caused during object release and how to address them:

  • Issues with uncollectable objects:

    • They often stay around until the Python process terminates, and thus can be considered memory leaks.
    • The garbage collector attempts to release them again and again on every collection run of its last generation, causing repeated unnecessary processing.

    In Python 3.4 or higher, the reasons for uncollectable objects have diminished very much and their presence usually indicates a bug. You should use tools that can detect uncollectable objects and then analyze each case to find out what caused the object to be uncollectable.

  • Issues with collected objects:

    • Increased processing overhead caused by the collector runs (compared to immediate release based on reference counting).
    • Suspension of all other activity in the Python process when the garbage collector runs.
    • The amount of memory bound to these objects until the garbage collector will run for the next time. Automatic runs of the garbage collector are triggered by heuristics that are based on the number of objects and not on the amount of memory bound to these objects, so it is possible to have a small number of collectable objects with large amounts of memory allocated, that are still not triggering a garbage collector run.

    Suitable measures to address these issues with collected objects:

    • Redesign to avoid circular references.
    • Replacement of normal references with weak references to get rid of circular references. Using weak references requires to be able to handle the case where the referenced object is unexpectedly gone, which can be properly detected.
    • Manually triggering additional garbage collector runs via gc.collect(). There are very few cases though where this actually improves anything. One reasonable case might be to trigger a collection after application startup to release the many objects that have been used temporarily during configuration and initial startup processing.
  • Issues with the __del__() method on objects that are involved in circular references on Python before 3.4:

    • The __del__() methods are not invoked, so the resource cleanup designed to be done by them does not happen.
    • In addition, the objects become uncollectable.

    Suitable measures to address these issues with the __del__() method:

    • The use of context managers is a good alternative to the use of the __del__() method, particularly on Python before 3.4.

2.3. Circular reference examples and detection

This section shows some simple examples of circular references.

Let’s first look at a simple way to surface circular references. The approach is to create an object, make it unreachable, and check whether a run of the garbage collector releases an object. If the object is involved in circular references, its reference count will not drop to zero when it becomes unreachable, but the garbage collector will be able to break up the circular references and release it. If the object is not involved in circular references, it will be released when it becomes it unreachable, and the garbage collector does not have anything to do with it (even when it was tracked).

This is basically the approach Yagot uses, although in a more automated fashion.

$ python
>>> import gc
>>> gc.collect()   # Run full garbage collection to have a reference
0                  # No objects collected initially (in this simple case)
>>> obj = dict()
>>> len(gc.get_referrers(obj))
1                 # The dict object has one referrer (the 'obj' variable)
>>> obj['self'] = obj
>>> len(gc.get_referrers(obj))
2                 # The dict object now in addition has its 'self' item as a referrer
>>> gc.collect()
0                  # Still no new objects collected
>>> del obj        # The dict object becomes unreachable ...
>>> gc.collect()
1                  # ... and was released by the next garbage collection run

The interesting part happens during the del obj statement. The del obj statement removes the name obj from its namespace. That causes the reference count of the dict object to drop by one. Because of the circular reference back from its ‘self’ item, the reference count is still 1, so it will not be released at that point. The call to gc.collect() triggers a full garbage collection run on all generations, which successfully breaks up the circular reference and releases the object, as reported by its return value of 1.

Here are some examples for circular references. You can inspect them using the approach described above:

  • List with a self-referencing item. This is not really useful code, but just a simple way to demonstrate a circular reference:

    obj = list()
  • Class with a self-referencing attribute. Another simple example for demonstration purposes:

    class SelfRef(object):
        def __init__(self):
            self.ref = self
    obj = SelfRef()
  • A tree node class that knows its parent and children. This is a more practical example and is very similar to what is done in xml.dom.minidom:

    class Node(object):
        def __init__(self):
            self.parentNode = None
            self.childNodes = []
        def appendChild(self, node):
            node.parentNode = self
    obj = Node()

2.4. Tools

This section lists some tools that can be used to detect memory leaks, garbage objects, and memory usage in Python.

TODO: Write section