10.6 Supporting Cyclic Garbarge 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_GC and provide an
implementation of the tp_traverse handler. The computed
value of the tp_basicsize field must include
PyGC_HEAD_SIZE as well. 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.
Extra memory needed for the garbage collector. Container objects
must include this in the calculation of their tp_basicsize. If the
collector is disabled at compile time then this is
Constructors for container types must conform to two rules:
- The memory for the object must be allocated using
PyObject_New() or PyObject_VarNew().
- Once all the fields which may contain references to other
containers are initialized, it must call
- void PyObject_GC_Init(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.
Similarly, the deallocator for the object must conform to a similar
pair of rules:
- Before fields which refer to other containers are invalidated,
PyObject_GC_Fini() must be called.
- The object's memory must be deallocated using
- void PyObject_GC_Fini(PyObject *op)
Remove the object op from the set of container objects tracked
by the collector. Note that PyObject_GC_Init() 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.
Note: Any container which may be referenced from another
object reachable by the collector must itself be tracked by the
collector, so it is generally not safe to call this function
anywhere but in the object's deallocator.
The tp_traverse handler accepts a function parameter of this
- 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 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. If
visit returns a non-zero value then an error has occurred and
that value should be returned immediately.
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
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