10.9 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.
An example showing the use of these interfaces can be found in
``Supporting the Cycle
Extending and Embedding the Python
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
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:
- The memory for the object must be allocated using
PyObject_GC_New() or PyObject_GC_VarNew().
- Once all the fields which may contain references to other
containers are initialized, it must call
|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,
Analogous to PyObject_NewVar() but for container objects
with the Py_TPFLAGS_HAVE_GC flag set.
|PyVarObject * PyObject_GC_Resize(||PyVarObject *op, Py_ssize_t)|
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:
- Before fields which refer to other containers are invalidated,
PyObject_GC_UnTrack() must be called.
- The object's memory must be deallocated using
|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
- 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:
my_traverse(Noddy *self, visitproc visit, void *arg)
New in version 2.4.
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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