Doc/c-api/object.rst
.. highlight:: c
.. _object:
.. c:function:: PyObject* Py_GetConstant(unsigned int constant_id)
Get a :term:strong reference to a constant.
Set an exception and return NULL if constant_id is invalid.
constant_id must be one of these constant identifiers:
.. c:namespace:: NULL
======================================== ===== =========================
Constant Identifier Value Returned object
======================================== ===== =========================
.. c:macro:: Py_CONSTANT_NONE 0 :py:data:None
.. c:macro:: Py_CONSTANT_FALSE 1 :py:data:False
.. c:macro:: Py_CONSTANT_TRUE 2 :py:data:True
.. c:macro:: Py_CONSTANT_ELLIPSIS 3 :py:data:Ellipsis
.. c:macro:: Py_CONSTANT_NOT_IMPLEMENTED 4 :py:data:NotImplemented
.. c:macro:: Py_CONSTANT_ZERO 5 0
.. c:macro:: Py_CONSTANT_ONE 6 1
.. c:macro:: Py_CONSTANT_EMPTY_STR 7 ''
.. c:macro:: Py_CONSTANT_EMPTY_BYTES 8 b''
.. c:macro:: Py_CONSTANT_EMPTY_TUPLE 9 ()
======================================== ===== =========================
Numeric values are only given for projects which cannot use the constant identifiers.
.. versionadded:: 3.13
.. impl-detail::
In CPython, all of these constants are :term:`immortal`.
.. c:function:: PyObject* Py_GetConstantBorrowed(unsigned int constant_id)
Similar to :c:func:Py_GetConstant, but return a :term:borrowed reference.
This function is primarily intended for backwards compatibility:
using :c:func:Py_GetConstant is recommended for new code.
The reference is borrowed from the interpreter, and is valid until the interpreter finalization.
.. versionadded:: 3.13
.. c:var:: PyObject* Py_NotImplemented
The NotImplemented singleton, used to signal that an operation is
not implemented for the given type combination.
.. c:macro:: Py_RETURN_NOTIMPLEMENTED
Properly handle returning :c:data:Py_NotImplemented from within a C
function (that is, create a new :term:strong reference
to :const:NotImplemented and return it).
.. c:macro:: Py_PRINT_RAW
Flag to be used with multiple functions that print the object (like
:c:func:PyObject_Print and :c:func:PyFile_WriteObject).
If passed, these functions use the :func:str of the object
instead of the :func:repr.
.. c:function:: int PyObject_Print(PyObject *o, FILE *fp, int flags)
Print an object o, on file fp. Returns -1 on error. The flags argument
is used to enable certain printing options. The only option currently supported
is :c:macro:Py_PRINT_RAW; if given, the :func:str of the object is written
instead of the :func:repr.
.. c:function:: void PyObject_Dump(PyObject *op)
Dump an object op to stderr. This should only be used for debugging.
The output is intended to try dumping objects even after memory corruption:
attached thread state, but
it's not recommended to do so: it can cause deadlocks.Example of output:
.. code-block:: output
object address : 0x7f80124702c0
object refcount : 2
object type : 0x9902e0
object type name: str
object repr : 'abcdef'
.. versionadded:: 3.15
.. c:function:: int PyObject_HasAttrWithError(PyObject *o, PyObject *attr_name)
Returns 1 if o has the attribute attr_name, and 0 otherwise.
This is equivalent to the Python expression hasattr(o, attr_name).
On failure, return -1.
.. versionadded:: 3.13
.. c:function:: int PyObject_HasAttrStringWithError(PyObject *o, const char *attr_name)
This is the same as :c:func:PyObject_HasAttrWithError, but attr_name is
specified as a :c:expr:const char* UTF-8 encoded bytes string,
rather than a :c:expr:PyObject*.
.. versionadded:: 3.13
.. c:function:: int PyObject_HasAttr(PyObject *o, PyObject *attr_name)
Returns 1 if o has the attribute attr_name, and 0 otherwise.
This function always succeeds.
.. note::
Exceptions that occur when this calls :meth:`~object.__getattr__` and
:meth:`~object.__getattribute__` methods aren't propagated,
but instead given to :func:`sys.unraisablehook`.
For proper error handling, use :c:func:`PyObject_HasAttrWithError`,
:c:func:`PyObject_GetOptionalAttr` or :c:func:`PyObject_GetAttr` instead.
.. c:function:: int PyObject_HasAttrString(PyObject *o, const char *attr_name)
This is the same as :c:func:PyObject_HasAttr, but attr_name is
specified as a :c:expr:const char* UTF-8 encoded bytes string,
rather than a :c:expr:PyObject*.
.. note::
Exceptions that occur when this calls :meth:`~object.__getattr__` and
:meth:`~object.__getattribute__` methods or while creating the temporary
:class:`str` object are silently ignored.
For proper error handling, use :c:func:`PyObject_HasAttrStringWithError`,
:c:func:`PyObject_GetOptionalAttrString`
or :c:func:`PyObject_GetAttrString` instead.
.. c:function:: PyObject* PyObject_GetAttr(PyObject *o, PyObject *attr_name)
Retrieve an attribute named attr_name from object o. Returns the attribute
value on success, or NULL on failure. This is the equivalent of the Python
expression o.attr_name.
If the missing attribute should not be treated as a failure, you can use
:c:func:PyObject_GetOptionalAttr instead.
.. c:function:: PyObject* PyObject_GetAttrString(PyObject *o, const char *attr_name)
This is the same as :c:func:PyObject_GetAttr, but attr_name is
specified as a :c:expr:const char* UTF-8 encoded bytes string,
rather than a :c:expr:PyObject*.
If the missing attribute should not be treated as a failure, you can use
:c:func:PyObject_GetOptionalAttrString instead.
.. c:function:: int PyObject_GetOptionalAttr(PyObject *obj, PyObject *attr_name, PyObject **result);
Variant of :c:func:PyObject_GetAttr which doesn't raise
:exc:AttributeError if the attribute is not found.
If the attribute is found, return 1 and set *result to a new
:term:strong reference to the attribute.
If the attribute is not found, return 0 and set *result to NULL;
the :exc:AttributeError is silenced.
If an error other than :exc:AttributeError is raised, return -1 and
set *result to NULL.
.. versionadded:: 3.13
.. c:function:: int PyObject_GetOptionalAttrString(PyObject *obj, const char *attr_name, PyObject **result);
This is the same as :c:func:PyObject_GetOptionalAttr, but attr_name is
specified as a :c:expr:const char* UTF-8 encoded bytes string,
rather than a :c:expr:PyObject*.
.. versionadded:: 3.13
.. c:function:: PyObject* PyObject_GenericGetAttr(PyObject *o, PyObject *name)
Generic attribute getter function that is meant to be put into a type
object's tp_getattro slot. It looks for a descriptor in the dictionary
of classes in the object's MRO as well as an attribute in the object's
:attr:~object.__dict__ (if present). As outlined in :ref:descriptors,
data descriptors take preference over instance attributes, while non-data
descriptors don't. Otherwise, an :exc:AttributeError is raised.
.. c:function:: int PyObject_SetAttr(PyObject *o, PyObject *attr_name, PyObject *v)
Set the value of the attribute named attr_name, for object o, to the value
v. Raise an exception and return -1 on failure;
return 0 on success. This is the equivalent of the Python statement
o.attr_name = v.
If v is NULL, the attribute is deleted. This behaviour is deprecated
in favour of using :c:func:PyObject_DelAttr, but there are currently no
plans to remove it.
The function must not be called with a NULL v and an exception set.
This case can arise from forgetting NULL checks and would delete the
attribute.
.. versionchanged:: 3.15 Must not be called with NULL value if an exception is set.
.. c:function:: int PyObject_SetAttrString(PyObject *o, const char *attr_name, PyObject *v)
This is the same as :c:func:PyObject_SetAttr, but attr_name is
specified as a :c:expr:const char* UTF-8 encoded bytes string,
rather than a :c:expr:PyObject*.
If v is NULL, the attribute is deleted, but this feature is
deprecated in favour of using :c:func:PyObject_DelAttrString.
The function must not be called with a NULL v and an exception set.
This case can arise from forgetting NULL checks and would delete the
attribute.
The number of different attribute names passed to this function
should be kept small, usually by using a statically allocated string
as attr_name.
For attribute names that aren't known at compile time, prefer calling
:c:func:PyUnicode_FromString and :c:func:PyObject_SetAttr directly.
For more details, see :c:func:PyUnicode_InternFromString, which may be
used internally to create a key object.
.. versionchanged:: 3.15 Must not be called with NULL value if an exception is set.
.. c:function:: int PyObject_GenericSetAttr(PyObject *o, PyObject *name, PyObject *value)
Generic attribute setter and deleter function that is meant
to be put into a type object's :c:member:~PyTypeObject.tp_setattro
slot. It looks for a data descriptor in the
dictionary of classes in the object's MRO, and if found it takes preference
over setting or deleting the attribute in the instance dictionary. Otherwise, the
attribute is set or deleted in the object's :attr:~object.__dict__ (if present).
On success, 0 is returned, otherwise an :exc:AttributeError
is raised and -1 is returned.
.. c:function:: int PyObject_DelAttr(PyObject *o, PyObject *attr_name)
Delete attribute named attr_name, for object o. Returns -1 on failure.
This is the equivalent of the Python statement del o.attr_name.
.. c:function:: int PyObject_DelAttrString(PyObject *o, const char *attr_name)
This is the same as :c:func:PyObject_DelAttr, but attr_name is
specified as a :c:expr:const char* UTF-8 encoded bytes string,
rather than a :c:expr:PyObject*.
The number of different attribute names passed to this function
should be kept small, usually by using a statically allocated string
as attr_name.
For attribute names that aren't known at compile time, prefer calling
:c:func:PyUnicode_FromString and :c:func:PyObject_DelAttr directly.
For more details, see :c:func:PyUnicode_InternFromString, which may be
used internally to create a key object for lookup.
.. c:function:: PyObject* PyObject_GenericGetDict(PyObject *o, void *context)
A generic implementation for the getter of a __dict__ descriptor. It
creates the dictionary if necessary.
This function may also be called to get the :py:attr:~object.__dict__
of the object o. Pass NULL for context when calling it.
Since this function may need to allocate memory for the
dictionary, it may be more efficient to call :c:func:PyObject_GetAttr
when accessing an attribute on the object.
On failure, returns NULL with an exception set.
.. versionadded:: 3.3
.. c:function:: int PyObject_GenericSetDict(PyObject *o, PyObject *value, void *context)
A generic implementation for the setter of a __dict__ descriptor. This
implementation does not allow the dictionary to be deleted.
.. versionadded:: 3.3
.. c:function:: PyObject** _PyObject_GetDictPtr(PyObject *obj)
Return a pointer to :py:attr:~object.__dict__ of the object obj.
If there is no __dict__, return NULL without setting an exception.
This function may need to allocate memory for the
dictionary, so it may be more efficient to call :c:func:PyObject_GetAttr
when accessing an attribute on the object.
.. c:function:: PyObject* PyObject_RichCompare(PyObject *o1, PyObject *o2, int opid)
Compare the values of o1 and o2 using the operation specified by opid,
which must be one of :c:macro:Py_LT, :c:macro:Py_LE, :c:macro:Py_EQ,
:c:macro:Py_NE, :c:macro:Py_GT, or :c:macro:Py_GE, corresponding to <,
<=, ==, !=, >, or >= respectively. This is the equivalent of
the Python expression o1 op o2, where op is the operator corresponding
to opid. Returns the value of the comparison on success, or NULL on failure.
.. c:function:: int PyObject_RichCompareBool(PyObject *o1, PyObject *o2, int opid)
Compare the values of o1 and o2 using the operation specified by opid,
like :c:func:PyObject_RichCompare, but returns -1 on error, 0 if
the result is false, 1 otherwise.
.. note::
If o1 and o2 are the same object, :c:func:PyObject_RichCompareBool
will always return 1 for :c:macro:Py_EQ and 0 for :c:macro:Py_NE.
.. c:function:: PyObject* PyObject_Format(PyObject *obj, PyObject *format_spec)
Format obj using format_spec. This is equivalent to the Python
expression format(obj, format_spec).
format_spec may be NULL. In this case the call is equivalent
to format(obj).
Returns the formatted string on success, NULL on failure.
.. c:function:: PyObject* PyObject_Repr(PyObject *o)
.. index:: pair: built-in function; repr
Compute a string representation of object o. Returns the string
representation on success, NULL on failure. This is the equivalent of the
Python expression repr(o). Called by the :func:repr built-in function.
If argument is NULL, return the string '<NULL>'.
.. versionchanged:: 3.4 This function now includes a debug assertion to help ensure that it does not silently discard an active exception.
.. c:function:: PyObject* PyObject_ASCII(PyObject *o)
.. index:: pair: built-in function; ascii
As :c:func:PyObject_Repr, compute a string representation of object o, but
escape the non-ASCII characters in the string returned by
:c:func:PyObject_Repr with \x, \u or \U escapes. This generates
a string similar to that returned by :c:func:PyObject_Repr in Python 2.
Called by the :func:ascii built-in function.
If argument is NULL, return the string '<NULL>'.
.. index:: string; PyObject_Str (C function)
.. c:function:: PyObject* PyObject_Str(PyObject *o)
Compute a string representation of object o. Returns the string
representation on success, NULL on failure. This is the equivalent of the
Python expression str(o). Called by the :func:str built-in function
and, therefore, by the :func:print function.
If argument is NULL, return the string '<NULL>'.
.. versionchanged:: 3.4 This function now includes a debug assertion to help ensure that it does not silently discard an active exception.
.. c:function:: PyObject* PyObject_Bytes(PyObject *o)
.. index:: pair: built-in function; bytes
Compute a bytes representation of object o. NULL is returned on
failure and a bytes object on success. This is equivalent to the Python
expression bytes(o), when o is not an integer. Unlike bytes(o),
a TypeError is raised when o is an integer instead of a zero-initialized
bytes object.
If argument is NULL, return the :class:bytes object b'<NULL>'.
.. c:function:: int PyObject_IsSubclass(PyObject *derived, PyObject *cls)
Return 1 if the class derived is identical to or derived from the class
cls, otherwise return 0. In case of an error, return -1.
If cls is a tuple, the check will be done against every entry in cls.
The result will be 1 when at least one of the checks returns 1,
otherwise it will be 0.
If cls has a :meth:~type.__subclasscheck__ method, it will be called to
determine the subclass status as described in :pep:3119. Otherwise,
derived is a subclass of cls if it is a direct or indirect subclass,
i.e. contained in :attr:cls.__mro__ <type.__mro__>.
Normally only class objects, i.e. instances of :class:type or a derived
class, are considered classes. However, objects can override this by having
a :attr:~type.__bases__ attribute (which must be a tuple of base classes).
.. c:function:: int PyObject_IsInstance(PyObject *inst, PyObject *cls)
Return 1 if inst is an instance of the class cls or a subclass of
cls, or 0 if not. On error, returns -1 and sets an exception.
If cls is a tuple, the check will be done against every entry in cls.
The result will be 1 when at least one of the checks returns 1,
otherwise it will be 0.
If cls has a :meth:~type.__instancecheck__ method, it will be called to
determine the subclass status as described in :pep:3119. Otherwise, inst
is an instance of cls if its class is a subclass of cls.
An instance inst can override what is considered its class by having a
:attr:~object.__class__ attribute.
An object cls can override if it is considered a class, and what its base
classes are, by having a :attr:~type.__bases__ attribute (which must be a tuple
of base classes).
.. c:function:: Py_hash_t PyObject_Hash(PyObject *o)
.. index:: pair: built-in function; hash
Compute and return the hash value of an object o. On failure, return -1.
This is the equivalent of the Python expression hash(o).
.. versionchanged:: 3.2
The return type is now Py_hash_t. This is a signed integer the same size
as :c:type:Py_ssize_t.
.. c:function:: Py_hash_t PyObject_HashNotImplemented(PyObject *o)
Set a :exc:TypeError indicating that type(o) is not :term:hashable and return -1.
This function receives special treatment when stored in a tp_hash slot,
allowing a type to explicitly indicate to the interpreter that it is not
hashable.
.. c:function:: int PyObject_IsTrue(PyObject *o)
Returns 1 if the object o is considered to be true, and 0 otherwise.
This is equivalent to the Python expression not not o. On failure, return
-1.
.. c:function:: int PyObject_Not(PyObject *o)
Returns 0 if the object o is considered to be true, and 1 otherwise.
This is equivalent to the Python expression not o. On failure, return
-1.
.. c:function:: PyObject* PyObject_Type(PyObject *o)
.. index:: pair: built-in function; type
When o is non-NULL, returns a type object corresponding to the object type
of object o. On failure, raises :exc:SystemError and returns NULL. This
is equivalent to the Python expression type(o).
This function creates a new :term:strong reference to the return value.
There's really no reason to use this
function instead of the :c:func:Py_TYPE() function, which returns a
pointer of type :c:expr:PyTypeObject*, except when a new
:term:strong reference is needed.
.. c:function:: int PyObject_TypeCheck(PyObject *o, PyTypeObject *type)
Return non-zero if the object o is of type type or a subtype of type, and
0 otherwise. Both parameters must be non-NULL.
.. c:function:: Py_ssize_t PyObject_Size(PyObject *o) Py_ssize_t PyObject_Length(PyObject *o)
.. index:: pair: built-in function; len
Return the length of object o. If the object o provides either the sequence
and mapping protocols, the sequence length is returned. On error, -1 is
returned. This is the equivalent to the Python expression len(o).
.. c:function:: Py_ssize_t PyObject_LengthHint(PyObject *o, Py_ssize_t defaultvalue)
Return an estimated length for the object o. First try to return its
actual length, then an estimate using :meth:~object.__length_hint__, and
finally return the default value. On error return -1. This is the
equivalent to the Python expression operator.length_hint(o, defaultvalue).
.. versionadded:: 3.4
.. c:function:: PyObject* PyObject_GetItem(PyObject *o, PyObject *key)
Return element of o corresponding to the object key or NULL on failure.
This is the equivalent of the Python expression o[key].
.. c:function:: int PyObject_SetItem(PyObject *o, PyObject *key, PyObject *v)
Map the object key to the value v. Raise an exception and
return -1 on failure; return 0 on success. This is the
equivalent of the Python statement o[key] = v. This function does
not steal a reference to v.
.. c:function:: int PyObject_DelItem(PyObject *o, PyObject *key)
Remove the mapping for the object key from the object o. Return -1
on failure. This is equivalent to the Python statement del o[key].
.. c:function:: int PyObject_DelItemString(PyObject *o, const char *key)
This is the same as :c:func:PyObject_DelItem, but key is
specified as a :c:expr:const char* UTF-8 encoded bytes string,
rather than a :c:expr:PyObject*.
.. c:function:: PyObject* PyObject_Dir(PyObject *o)
This is equivalent to the Python expression dir(o), returning a (possibly
empty) list of strings appropriate for the object argument, or NULL if there
was an error. If the argument is NULL, this is like the Python dir(),
returning the names of the current locals; in this case, if no execution frame
is active then NULL is returned but :c:func:PyErr_Occurred will return false.
.. c:function:: PyObject* PyObject_GetIter(PyObject *o)
This is equivalent to the Python expression iter(o). It returns a new
iterator for the object argument, or the object itself if the object is already
an iterator. Raises :exc:TypeError and returns NULL if the object cannot be
iterated.
.. c:function:: PyObject* PyObject_SelfIter(PyObject *obj)
This is equivalent to the Python __iter__(self): return self method.
It is intended for :term:iterator types, to be used in the :c:member:PyTypeObject.tp_iter slot.
.. c:function:: PyObject* PyObject_GetAIter(PyObject *o)
This is the equivalent to the Python expression aiter(o). Takes an
:class:AsyncIterable object and returns an :class:AsyncIterator for it.
This is typically a new iterator but if the argument is an
:class:AsyncIterator, this returns itself. Raises :exc:TypeError and
returns NULL if the object cannot be iterated.
.. versionadded:: 3.10
.. c:function:: void *PyObject_GetTypeData(PyObject *o, PyTypeObject *cls)
Get a pointer to subclass-specific data reserved for cls.
The object o must be an instance of cls, and cls must have been
created using negative :c:member:PyType_Spec.basicsize.
Python does not check this.
On error, set an exception and return NULL.
.. versionadded:: 3.12
.. c:function:: Py_ssize_t PyType_GetTypeDataSize(PyTypeObject *cls)
Return the size of the instance memory space reserved for cls, i.e. the size of the
memory :c:func:PyObject_GetTypeData returns.
This may be larger than requested using :c:member:-PyType_Spec.basicsize <PyType_Spec.basicsize>;
it is safe to use this larger size (e.g. with :c:func:!memset).
The type cls must have been created using
negative :c:member:PyType_Spec.basicsize.
Python does not check this.
On error, set an exception and return a negative value.
.. versionadded:: 3.12
.. c:function:: void *PyObject_GetItemData(PyObject *o)
Get a pointer to per-item data for a class with
:c:macro:Py_TPFLAGS_ITEMS_AT_END.
On error, set an exception and return NULL.
:py:exc:TypeError is raised if o does not have
:c:macro:Py_TPFLAGS_ITEMS_AT_END set.
.. versionadded:: 3.12
.. c:function:: int PyObject_VisitManagedDict(PyObject *obj, visitproc visit, void *arg)
Visit the managed dictionary of obj.
This function must only be called in a traverse function of the type which
has the :c:macro:Py_TPFLAGS_MANAGED_DICT flag set.
.. versionadded:: 3.13
.. c:function:: void PyObject_ClearManagedDict(PyObject *obj)
Clear the managed dictionary of obj.
This function must only be called in a clear function of the type which
has the :c:macro:Py_TPFLAGS_MANAGED_DICT flag set.
.. versionadded:: 3.13
.. c:function:: int PyUnstable_Object_EnableDeferredRefcount(PyObject *obj)
Enable deferred reference counting <https://peps.python.org/pep-0703/#deferred-reference-counting>_ on obj,
if supported by the runtime. In the :term:free-threaded <free threading> build,
this allows the interpreter to avoid reference count adjustments to obj,
which may improve multi-threaded performance. The tradeoff is
that obj will only be deallocated by the tracing garbage collector, and
not when the interpreter no longer has any references to it.
This function returns 1 if deferred reference counting is enabled on obj,
and 0 if deferred reference counting is not supported or if the hint was
ignored by the interpreter, such as when deferred reference counting is already
enabled on obj. This function is thread-safe, and cannot fail.
This function does nothing on builds with the :term:GIL enabled, which do
not support deferred reference counting. This also does nothing if obj is not
an object tracked by the garbage collector (see :func:gc.is_tracked and
:c:func:PyObject_GC_IsTracked).
This function is intended to be used soon after obj is created,
by the code that creates it, such as in the object's :c:member:~PyTypeObject.tp_new
slot.
.. versionadded:: 3.14
.. c:function:: int PyUnstable_Object_IsUniqueReferencedTemporary(PyObject *obj)
Check if obj is a unique temporary object.
Returns 1 if obj is known to be a unique temporary object,
and 0 otherwise. This function cannot fail, but the check is
conservative, and may return 0 in some cases even if obj is a unique
temporary object.
If an object is a unique temporary, it is guaranteed that the current code
has the only reference to the object. For arguments to C functions, this
should be used instead of checking if the reference count is 1. Starting
with Python 3.14, the interpreter internally avoids some reference count
modifications when loading objects onto the operands stack by
:term:borrowing <borrowed reference> references when possible, which means
that a reference count of 1 by itself does not guarantee that a function
argument uniquely referenced.
In the example below, my_func is called with a unique temporary object
as its argument::
my_func([1, 2, 3])
In the example below, my_func is not called with a unique temporary
object as its argument, even if its refcount is 1::
my_list = [1, 2, 3]
my_func(my_list)
See also the function :c:func:Py_REFCNT.
.. versionadded:: 3.14
.. c:function:: int PyUnstable_IsImmortal(PyObject *obj)
This function returns non-zero if obj is :term:immortal, and zero
otherwise. This function cannot fail.
.. note::
Objects that are immortal in one CPython version are not guaranteed to
be immortal in another.
.. versionadded:: 3.14
.. c:function:: int PyUnstable_TryIncRef(PyObject *obj)
Increments the reference count of obj if it is not zero. Returns 1
if the object's reference count was successfully incremented. Otherwise,
this function returns 0.
:c:func:PyUnstable_EnableTryIncRef must have been called
earlier on obj or this function may spuriously return 0 in the
:term:free-threaded build.
This function is logically equivalent to the following C code, except that
it behaves atomically in the :term:free-threaded build::
if (Py_REFCNT(op) > 0) {
Py_INCREF(op);
return 1;
}
return 0;
This is intended as a building block for managing weak references
without the overhead of a Python :ref:weak reference object <weakrefobjects>.
Typically, correct use of this function requires support from obj's
deallocator (:c:member:~PyTypeObject.tp_dealloc).
For example, the following sketch could be adapted to implement a
"weakmap" that works like a :py:class:~weakref.WeakValueDictionary
for a specific type:
.. code-block:: c
PyMutex mutex;
PyObject *
add_entry(weakmap_key_type *key, PyObject *value)
{
PyUnstable_EnableTryIncRef(value);
weakmap_type weakmap = ...;
PyMutex_Lock(&mutex);
weakmap_add_entry(weakmap, key, value);
PyMutex_Unlock(&mutex);
Py_RETURN_NONE;
}
PyObject *
get_value(weakmap_key_type *key)
{
weakmap_type weakmap = ...;
PyMutex_Lock(&mutex);
PyObject *result = weakmap_find(weakmap, key);
if (PyUnstable_TryIncRef(result)) {
// `result` is safe to use
PyMutex_Unlock(&mutex);
return result;
}
// if we get here, `result` is starting to be garbage-collected,
// but has not been removed from the weakmap yet
PyMutex_Unlock(&mutex);
return NULL;
}
// tp_dealloc function for weakmap values
void
value_dealloc(PyObject *value)
{
weakmap_type weakmap = ...;
PyMutex_Lock(&mutex);
weakmap_remove_value(weakmap, value);
...
PyMutex_Unlock(&mutex);
}
.. versionadded:: 3.14
.. c:function:: void PyUnstable_EnableTryIncRef(PyObject *obj)
Enables subsequent uses of :c:func:PyUnstable_TryIncRef on obj. The
caller must hold a :term:strong reference to obj when calling this.
.. versionadded:: 3.14
.. c:function:: int PyUnstable_Object_IsUniquelyReferenced(PyObject *op)
Determine if op only has one reference.
On GIL-enabled builds, this function is equivalent to
:c:expr:Py_REFCNT(op) == 1.
On a :term:free-threaded build, this checks if op's
:term:reference count is equal to one and additionally checks if op
is only used by this thread. :c:expr:Py_REFCNT(op) == 1 is not
thread-safe on free-threaded builds; prefer this function.
The caller must hold an :term:attached thread state, despite the fact
that this function doesn't call into the Python interpreter. This function
cannot fail.
.. versionadded:: 3.14
.. c:function:: int PyUnstable_SetImmortal(PyObject *op)
Marks the object op :term:immortal. The argument should be uniquely referenced by
the calling thread. This is intended to be used for reducing reference counting contention
in the :term:free-threaded build for objects which are shared across threads.
This is a one-way process: objects can only be made immortal; they cannot be made mortal once again. Immortal objects do not participate in reference counting and will never be garbage collected. If the object is GC-tracked, it is untracked.
This function is intended to be used soon after op is created, by the code that
creates it, such as in the object's :c:member:~PyTypeObject.tp_new slot.
Returns 1 if the object was made immortal and returns 0 if it was not.
This function cannot fail.
.. versionadded:: 3.15