Definition slots

To define module objects and classes using the C API, you may use an array of slots – essentally, key-value pairs that describe features of the object to create. This decouples the data from the structures used at runtime, allowing CPython – and other Python C API implementations – to update the stuctures without breaking backwards compatibility.

This section documents slots in general. For object-specific behavior and slot values, see documentation for functions that apply slots:

• PyType_FromSlots() for types;

• PyModule_FromSlotsAndSpec() and Extension export hook for modules.

When slots are passed to a function that applies them, the function will not modify the slot array, nor any data it points to (recursively). After the function is done, the caller is allowed to modify or deallocate the array and any data it points to (recursively), except data explicitly marked with PySlot_STATIC.

Except when documented otherwise, multiple slots with the same ID (sl_id) may not occur in a single slots array.

Added in version 3.15.0a8 (unreleased): Slot arrays generalize an earlier way of defining objects: using PyType_Spec with PyType_Slot for types, and PyModuleDef with PyModuleDef_Slot for modules. The earlier API is soft deprecated; there are no plans to remove it.

Entries of the slots array use the following structure:

type PySlot

Part of the Stable ABI (including all members) since version 3.15.

An entry in a slots array. Defined as:

typedef struct {
    uint16_t sl_id;
    uint16_t sl_flags;
    uint32_t _reserved;  // must be 0
    union {
        void *sl_ptr;
        void (*sl_func)(void);
        Py_ssize_t sl_size;
        int64_t sl_int64;
        uint64_t sl_uint64;
    };
} PySlot;

uint16_t sl_id

A slot ID, chosen from:

• Py_slot_* values documented in Common slot IDs below;

• Py_mod_* values for modules, as documented in Module definition;

• Values for types, as documented in Type slot IDs.

A sl_id of zero (Py_slot_end) marks the end of a slots array.

void *sl_ptr

void (*sl_func)(void)

Py_ssize_t sl_size

int64_t sl_int64

uint64_t sl_uint64

The data for the slot. These members are part of an anonymous union; the member to use depends on which data type is required by the slot ID: data pointer, function pointer, size, signed or unsigned integer, respectively.

Except when documented otherwise for a specific slot ID, pointers (that is sl_ptr and sl_func) may not be NULL.

uint16_t sl_flags

Zero or more of the following flags, OR-ed together:

PySlot_STATIC

Part of the Stable ABI since version 3.15.

All data the slot points to is statically allocated and constant. Thus, the interpreter does not need to copy the information.

This flag is implied for function pointers.

The flag applies even to data the slot points to “indirectly”, except for slots nested via Py_slot_subslots which may have their own PySlot_STATIC` flags. For example, if applied to a Py_tp_members slot that points to an array of PyMemberDef structures, then the entire array, as well as the name and doc strings in its elements, must be static and constant.

PySlot_INTPTR

Part of the Stable ABI since version 3.15.

The data is stored in sl_ptr; CPython will cast it to the appropriate type.

This flag can simplify porting from the older PyType_Slot and PyModuleDef_Slot structures.

PySlot_OPTIONAL

Part of the Stable ABI since version 3.15.

If the slot ID is unknown, the interpreter should ignore the slot, rather than fail.

For example, if Python 3.16 adds a new feature with a new slot ID,attr the corresponding slot may be marked PySlot_OPTIONAL so that Python 3.15 ignores it.

Note that the “optionality” only applies to unknown slot IDs. This flag does not make Python skip invalid values of known slots.

Added in version 3.15.0a8 (unreleased).

Convenience macros

PySlot_DATA(name, value)

PySlot_FUNC(name, value)

PySlot_SIZE(name, value)

PySlot_INT64(name, value)

PySlot_UINT64(name, value)

PySlot_STATIC_DATA(name, value)

Part of the Stable ABI since version 3.15.

Convenience macros to define PySlot structures with sl_id and a particular union member set.

PySlot_STATIC_DATA sets the PySlot_STATIC flag; others set no flags.

Note that these macros use designated initializers, a C language feature that C++ added in the 2000 version of the standard. If your code needs to be compatible with C++11 or older, use PySlot_PTR instead.

Defined as:

#define PySlot_DATA(NAME, VALUE) \
   {.sl_id=NAME, .sl_ptr=(void*)(VALUE)}

#define PySlot_FUNC(NAME, VALUE) \
   {.sl_id=NAME, .sl_func=(VALUE)}

#define PySlot_SIZE(NAME, VALUE) \
   {.sl_id=NAME, .sl_size=(VALUE)}

#define PySlot_INT64(NAME, VALUE) \
   {.sl_id=NAME, .sl_int64=(VALUE)}

#define PySlot_UINT64(NAME, VALUE) \
   {.sl_id=NAME, .sl_uint64=(VALUE)}

#define PySlot_STATIC_DATA(NAME, VALUE) \
   {.sl_id=NAME, .sl_flags=PySlot_STATIC, .sl_ptr=(VALUE)}

Added in version 3.15.0a8 (unreleased).

PySlot_END

Part of the Stable ABI since version 3.15.

Convenience macros to mark the end of a PySlot array.

Defined as:

#define PySlot_END {0}

Added in version 3.15.0a8 (unreleased).

PySlot_PTR(name, value)

PySlot_PTR_STATIC(name, value)

Part of the Stable ABI since version 3.15.

Convenience macros for use in C++11-compatible code. This version of C++ does not allow setting arbitrary union members in literals, these macros set the PySlot_INTPTR flag and cast the value to (void*).

Defined as:

#define PySlot_PTR(NAME, VALUE) \
   {NAME, PySlot_INTPTR, {0}, {(void*)(VALUE)}}

#define PySlot_PTR_STATIC(NAME, VALUE) \
   {NAME, PySlot_INTPTR|Py_SLOT_STATIC, {0}, {(void*)(VALUE)}}

Added in version 3.15.0a8 (unreleased).

Common slot IDs

The following slot IDs may be used in both type and module definitions.

Py_slot_end

Part of the Stable ABI since version 3.15.

Marks the end of a slots array. Defined as zero.

Added in version 3.15.0a8 (unreleased).

Py_slot_subslots

Part of the Stable ABI since version 3.15.

Nested slots array.

The value (sl_ptr) should point to an array of PySlot structures. The slots in the array (up to but not including the zero-ID terminator) will be treated as if they were inserted if the current slot array, at the point Py_slot_subslots appears.

Slot nesting depth is limited to 5 levels. This restriction may be lifted in the future.

Added in version 3.15.0a8 (unreleased).

Py_slot_invalid

Part of the Stable ABI since version 3.15.

Reserved; will always be treated as an unknown slot ID. Defined as UINT16_MAX (0xFFFF).

When used with the PySlot_OPTIONAL flag, defines a slot with no effect. Without the flag, processing a slot with this ID will fail.

Added in version 3.15.0a8 (unreleased).