# 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.

  New 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.

  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)`).

PyObject *`PyFrozenSet_New`(PyObject *iterable)
  Return value: New reference. Part of the Stable ABI.

  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.

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.

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

Py_ssize_t `PySet_GET_SIZE`(PyObject *anyset)

  Macro form of `PySet_Size()` without error checking.

int `PySet_Contains`(PyObject *anyset, PyObject *key)
  Part of the Stable ABI.

  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 `PyExc_SystemError` if anyset is not a `set`, `frozenset`, or an instance of a subtype.

int `PySet_Add`(PyObject *set, PyObject *key)
  Part of the Stable ABI.

  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.

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.

  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 `PyExc_SystemError` if set is not an instance of `set` or its subtype.

PyObject *`PySet_Pop`(PyObject *set)
  Return value: New reference. Part of the Stable ABI.

  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.

  Empty an existing set of all elements.