Set Objects

This section details the public API for set and frozenset objects. Any functionality not listed below is best accessed using either the abstract object protocol (including PyObject_CallMethod(), PyObject_RichCompareBool(), PyObject_Hash(), PyObject_Repr(), PyObject_IsTrue(), PyObject_Print(), and PyObject_GetIter()) or the abstract number protocol (including PyNumber_And(), PyNumber_Subtract(), PyNumber_Or(), PyNumber_Xor(), PyNumber_InPlaceAnd(), PyNumber_InPlaceSubtract(), PyNumber_InPlaceOr(), and PyNumber_InPlaceXor()).

type PySetObject

This subtype of PyObject is used to hold the internal data for both set and frozenset objects. It is like a PyDictObject in that it is a fixed size for small sets (much like tuple storage) and will point to a separate, variable sized block of memory for medium and large sized sets (much like list storage). None of the fields of this structure should be considered public and all are subject to change. All access should be done through the documented API rather than by manipulating the values in the structure.

PyTypeObject PySet_Type

Part of the Stable ABI.

This is an instance of PyTypeObject representing the Python set type.

PyTypeObject PyFrozenSet_Type

Part of the Stable ABI.

This is an instance of PyTypeObject representing the Python frozenset type.

The following type check macros work on pointers to any Python object. Likewise, the constructor functions work with any iterable Python object.

int PySet_Check(PyObject *p)

Return true if p is a set object or an instance of a subtype. This function always succeeds.

int PyFrozenSet_Check(PyObject *p)

Return true if p is a frozenset object or an instance of a subtype. This function always succeeds.

int PyAnySet_Check(PyObject *p)

Return true if p is a set object, a frozenset object, or an instance of a subtype. This function always succeeds.

int PySet_CheckExact(PyObject *p)

Return true if p is a set object but not an instance of a subtype. This function always succeeds.

Added in version 3.10.

int PyAnySet_CheckExact(PyObject *p)

Return true if p is a set object or a frozenset object but not an instance of a subtype. This function always succeeds.

int PyFrozenSet_CheckExact(PyObject *p)

Return true if p is a frozenset object but not an instance of a subtype. This function always succeeds.

PyObject *PySet_New(PyObject *iterable)

Return value: New reference. Part of the Stable ABI. Thread safety: Safe for concurrent use on the same object.

Return a new set containing objects returned by the iterable. The iterable may be NULL to create a new empty set. Return the new set on success or NULL on failure. Raise TypeError if iterable is not actually iterable. The constructor is also useful for copying a set (c=set(s)).

Note

The operation is atomic on free threading when iterable is a set, frozenset, dict or frozendict.

PyObject *PyFrozenSet_New(PyObject *iterable)

Return value: New reference. Part of the Stable ABI. Thread safety: Safe for concurrent use on the same object.

Return a new frozenset containing objects returned by the iterable. The iterable may be NULL to create a new empty frozenset. Return the new set on success or NULL on failure. Raise TypeError if iterable is not actually iterable.

See also the PyFrozenSetWriter API.

Note

The operation is atomic on free threading when iterable is a set, frozenset, dict or frozendict.

The following functions and macros are available for instances of set or frozenset or instances of their subtypes.

Py_ssize_t PySet_Size(PyObject *anyset)

Part of the Stable ABI. Thread safety: Atomic.

Return the length of a set or frozenset object. Equivalent to len(anyset). Raises a SystemError if anyset is not a set, frozenset, or an instance of a subtype.

Py_ssize_t PySet_GET_SIZE(PyObject *anyset)

Thread safety: Atomic.

Macro form of PySet_Size() without error checking.

int PySet_Contains(PyObject *anyset, PyObject *key)

Part of the Stable ABI. Thread safety: Safe for concurrent use on the same object.

Return 1 if found, 0 if not found, and -1 if an error is encountered. Unlike the Python __contains__() method, this function does not automatically convert unhashable sets into temporary frozensets. Raise a TypeError if the key is unhashable. Raise SystemError if anyset is not a set, frozenset, or an instance of a subtype.

Note

The operation is atomic on free threading when key is str, int, float, bool or bytes.

int PySet_Add(PyObject *set, PyObject *key)

Part of the Stable ABI. Thread safety: Safe for concurrent use on the same object.

Add key to a set instance. Also works with frozenset instances (like PyTuple_SetItem() it can be used to fill in the values of brand new frozensets before they are exposed to other code). Return 0 on success or -1 on failure. Raise a TypeError if the key is unhashable. Raise a MemoryError if there is no room to grow. Raise a SystemError if set is not an instance of set or its subtype.

Note

The operation is atomic on free threading when key is str, int, float, bool or bytes.

The following functions are available for instances of set or its subtypes but not for instances of frozenset or its subtypes.

int PySet_Discard(PyObject *set, PyObject *key)

Part of the Stable ABI. Thread safety: Safe for concurrent use on the same object.

Return 1 if found and removed, 0 if not found (no action taken), and -1 if an error is encountered. Does not raise KeyError for missing keys. Raise a TypeError if the key is unhashable. Unlike the Python discard() method, this function does not automatically convert unhashable sets into temporary frozensets. Raise SystemError if set is not an instance of set or its subtype.

Note

The operation is atomic on free threading when key is str, int, float, bool or bytes.

PyObject *PySet_Pop(PyObject *set)

Return value: New reference. Part of the Stable ABI. Thread safety: Atomic.

Return a new reference to an arbitrary object in the set, and removes the object from the set. Return NULL on failure. Raise KeyError if the set is empty. Raise a SystemError if set is not an instance of set or its subtype.

int PySet_Clear(PyObject *set)

Part of the Stable ABI. Thread safety: Atomic.

Empty an existing set of all elements. Return 0 on success. Return -1 and raise SystemError if set is not an instance of set or its subtype.

Note

In the free-threaded build, the set is emptied before its entries are cleared, so other threads will observe an empty set rather than intermediate states.

Deprecated API

PySet_MINSIZE

A constant representing the size of an internal preallocated table inside PySetObject instances.

This is documented solely for completeness, as there are no guarantees that a given version of CPython uses preallocated tables with a fixed size. In code that does not deal with unstable set internals, PySet_MINSIZE can be replaced with a small constant like 8.

If looking for the size of a set, use PySet_Size() instead.

Soft deprecated since version 3.14.

PyFrozenSetWriter

The PyFrozenSetWriter API can be used to create a Python frozenset object.

See also the PyFrozenSet_New() function.

Added in version 3.16.0a1 (unreleased).

type PyFrozenSetWriter

A frozenset writer object.

The API is not thread safe. To share a writer with multiple threads, a critical section or a lock is needed.

The instance must be destroyed by PyFrozenSetWriter_Finish() on success, or PyFrozenSetWriter_Discard() on error.

PyFrozenSetWriter *PyFrozenSetWriter_Create(void)

Create a PyFrozenSetWriter.

On error, set an exception and return NULL.

size must be positive or zero.

PyObject *PyFrozenSetWriter_Finish(PyFrozenSetWriter *writer)

Finish a PyFrozenSetWriter created by PyFrozenSetWriter_Create().

On success, return a Python frozenset object. On error, set an exception and return NULL.

The writer instance is invalid after the call in any case. No API can be called on the writer after PyFrozenSetWriter_Finish().

void PyFrozenSetWriter_Discard(PyFrozenSetWriter *writer)

Discard a PyFrozenSetWriter created by PyFrozenSetWriter_Create().

Do nothing if writer is NULL.

The writer instance is invalid after the call. No API can be called on the writer after PyFrozenSetWriter_Discard().

int PyFrozenSetWriter_Add(PyFrozenSetWriter *writer, PyObject *item)

Add item to a frozenset writer.

On success, return 0. On error, set an exception and return -1.

int PyFrozenSetWriter_Update(PyFrozenSetWriter *writer, PyObject *iterable)

Update the frozenset writer, adding elements from iterable.

On success, return 0. On error, set an exception and return -1.