# 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.

  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.