Python/ceval.c (part 65)

Source:

cpython 3.14 @ ab2d84fe1023/Python/ceval.c ↗

This annotation covers STORE_ATTR specialization. See python_ceval64_detail for CALL specializations.

Map

Lines	Symbol	Role
1-80	STORE_ATTR baseline	Generic attribute set via tp_setattro
81-160	STORE_ATTR_INSTANCE_VALUE	Fast path: write __dict__ slot by index
161-240	STORE_ATTR_SLOT	Write a __slots__ slot directly
241-340	STORE_ATTR_WITH_HINT	Dict write with cached key index
341-500	Type version invalidation	When STORE_ATTR deoptimizes

Reading

STORE_ATTR_INSTANCE_VALUE

// CPython: Python/ceval.c:2960 STORE_ATTR_INSTANCE_VALUE
inst(STORE_ATTR_INSTANCE_VALUE, (unused/4, owner, value --)) {
    uint16_t index = read_u16(&next_instr[-1].cache);
    uint32_t ep_version = read_u32(&next_instr[-3].cache);
    PyDictObject *dict = _PyObject_GetManagedDict(owner);
    DEOPT_IF(dict == NULL, STORE_ATTR);
    DEOPT_IF(dict->ma_version_tag != ep_version, STORE_ATTR);
    PyObject *old_value = dict->ma_values->values[index];
    dict->ma_values->values[index] = value;
    Py_XDECREF(old_value);
    _PyDict_NotifyEvent(tstate->interp, PyDict_EVENT_MODIFIED, dict, ...);
    Py_DECREF(owner);
}

STORE_ATTR_INSTANCE_VALUE writes to a known slot in the instance's split dict values array. The version tag check ensures the dict layout hasn't changed. On deopt, updates the type's inline cache and falls back to STORE_ATTR.

STORE_ATTR_SLOT

// CPython: Python/ceval.c:3020 STORE_ATTR_SLOT
inst(STORE_ATTR_SLOT, (unused/4, owner, value --)) {
    uint16_t index = read_u16(&next_instr[-1].cache);
    char *addr = (char *)owner + index;
    PyObject *old_value = *(PyObject **)addr;
    *(PyObject **)addr = value;
    Py_XDECREF(old_value);
    Py_DECREF(owner);
}

For __slots__ classes, attributes are stored at fixed offsets in the object's memory, not in a dict. STORE_ATTR_SLOT writes directly to (char *)owner + index. This is as fast as C struct field access.

Type version invalidation

// CPython: Objects/typeobject.c:460 type_modified_unlocked
void
type_modified_unlocked(PyTypeObject *type)
{
    /* Increment the type's version tag */
    if (type->tp_version_tag == 0) return;
    type->tp_version_tag = 0;  /* 0 means "invalid" */
    /* Invalidate all caches for this type and its subclasses */
    for each subtype in type->tp_subclasses:
        type_modified_unlocked(subtype);
}

Any attribute assignment to a type (adding a method, changing a class variable) calls type_modified_unlocked. This invalidates the tp_version_tag used by all LOAD_ATTR_* and STORE_ATTR_* specializations for that type. All specialized instructions for the type deopt on their next execution.

gopy notes

STORE_ATTR_INSTANCE_VALUE is in vm/eval_simple.go. It writes to objects.Instance.Values[index]. STORE_ATTR_SLOT writes to the slot offset in the Go struct via unsafe.Pointer. Type version tags are invalidated in objects/type.go when __setattr__ modifies a type.