Supporting Cyclic Garbage Collection

Python’s support for detecting and collecting garbage which involves circular references requires support from object types which are “containers” for other objects which may also be containers. Types which do not store references to other objects, or which only store references to atomic types (such as numbers or strings), do not need to provide any explicit support for garbage collection.

To create a container type, the tp_flags field of the type object must include the Py_TPFLAGS_HAVE_GC and provide an implementation of the tp_traverse handler. If instances of the type are mutable, a tp_clear implementation must also be provided.

Objects with a type with this flag set must conform with the rules documented here. For convenience these objects will be referred to as container objects.

Constructors for container types must conform to two rules:

  1. The memory for the object must be allocated using PyObject_GC_New() or PyObject_GC_NewVar().
  2. Once all the fields which may contain references to other containers are initialized, it must call PyObject_GC_Track().
TYPE* PyObject_GC_New(TYPE, PyTypeObject *type)
Analogous to PyObject_New() but for container objects with the Py_TPFLAGS_HAVE_GC flag set.
TYPE* PyObject_GC_NewVar(TYPE, PyTypeObject *type, Py_ssize_t size)
Analogous to PyObject_NewVar() but for container objects with the Py_TPFLAGS_HAVE_GC flag set.
TYPE* PyObject_GC_Resize(TYPE, PyVarObject *op, Py_ssize_t newsize)
Resize an object allocated by PyObject_NewVar(). Returns the resized object or NULL on failure.
void PyObject_GC_Track(PyObject *op)
Adds the object op to the set of container objects tracked by the collector. The collector can run at unexpected times so objects must be valid while being tracked. This should be called once all the fields followed by the tp_traverse handler become valid, usually near the end of the constructor.
void _PyObject_GC_TRACK(PyObject *op)
A macro version of PyObject_GC_Track(). It should not be used for extension modules.

Similarly, the deallocator for the object must conform to a similar pair of rules:

  1. Before fields which refer to other containers are invalidated, PyObject_GC_UnTrack() must be called.
  2. The object’s memory must be deallocated using PyObject_GC_Del().
void PyObject_GC_Del(void *op)
Releases memory allocated to an object using PyObject_GC_New() or PyObject_GC_NewVar().
void PyObject_GC_UnTrack(void *op)
Remove the object op from the set of container objects tracked by the collector. Note that PyObject_GC_Track() can be called again on this object to add it back to the set of tracked objects. The deallocator (tp_dealloc handler) should call this for the object before any of the fields used by the tp_traverse handler become invalid.
void _PyObject_GC_UNTRACK(PyObject *op)
A macro version of PyObject_GC_UnTrack(). It should not be used for extension modules.

The tp_traverse handler accepts a function parameter of this type:

int (*visitproc)(PyObject *object, void *arg)
Type of the visitor function passed to the tp_traverse handler. The function should be called with an object to traverse as object and the third parameter to the tp_traverse handler as arg. The Python core uses several visitor functions to implement cyclic garbage detection; it’s not expected that users will need to write their own visitor functions.

The tp_traverse handler must have the following type:

int (*traverseproc)(PyObject *self, visitproc visit, void *arg)
Traversal function for a container object. Implementations must call the visit function for each object directly contained by self, with the parameters to visit being the contained object and the arg value passed to the handler. The visit function must not be called with a NULL object argument. If visit returns a non-zero value that value should be returned immediately.

To simplify writing tp_traverse handlers, a Py_VISIT() macro is provided. In order to use this macro, the tp_traverse implementation must name its arguments exactly visit and arg:

void Py_VISIT(PyObject *o)

Call the visit callback, with arguments o and arg. If visit returns a non-zero value, then return it. Using this macro, tp_traverse handlers look like:

static int
my_traverse(Noddy *self, visitproc visit, void *arg)
    return 0;

The tp_clear handler must be of the inquiry type, or NULL if the object is immutable.

int (*inquiry)(PyObject *self)
Drop references that may have created reference cycles. Immutable objects do not have to define this method since they can never directly create reference cycles. Note that the object must still be valid after calling this method (don’t just call Py_DECREF() on a reference). The collector will call this method if it detects that this object is involved in a reference cycle.

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