集合对象
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This section details the public API for "set" and "frozenset" objects.
Any functionality not listed below is best accessed using the 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()").

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 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

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

PyTypeObject PyFrozenSet_Type

   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.

int PyFrozenSet_Check(PyObject *p)

   Return true if *p* is a "frozenset" object or an instance of a
   subtype.

int PyAnySet_Check(PyObject *p)

   Return true if *p* is a "set" object, a "frozenset" object, or an
   instance of a subtype.

int PyAnySet_CheckExact(PyObject *p)

   Return true if *p* is a "set" object or a "frozenset" object but
   not an instance of a subtype.

int PyFrozenSet_CheckExact(PyObject *p)

   Return true if *p* is a "frozenset" object but not an instance of a
   subtype.

PyObject* PySet_New(PyObject *iterable)
    *Return value: New reference.*

   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.*

   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)

   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)

   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)

   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)

   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.*

   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)

   清空现有字典的所有键值对。

int PySet_ClearFreeList()

   Clear the free list. Return the total number of freed items.

   3.3 新版功能.
