cpython/Include/cpython/setobject.h

#ifndef Py_CPYTHON_SETOBJECT_H
#  error "this header file must not be included directly"
#endif

/* There are three kinds of entries in the table:

1. Unused:  key == NULL and hash == 0
2. Dummy:   key == dummy and hash == -1
3. Active:  key != NULL and key != dummy and hash != -1

The hash field of Unused slots is always zero.

The hash field of Dummy slots are set to -1
meaning that dummy entries can be detected by
either entry->key==dummy or by entry->hash==-1.
*/

#define PySet_MINSIZE …

setentry;

/* The SetObject data structure is shared by set and frozenset objects.

Invariant for sets:
 - hash is -1

Invariants for frozensets:
 - data is immutable.
 - hash is the hash of the frozenset or -1 if not computed yet.

*/

PySetObject;

#define _PySet_CAST(so) …

static inline Py_ssize_t PySet_GET_SIZE(PyObject *so) { … }
#define PySet_GET_SIZE(so) …