Objects/unionobject.c (part 2)

Source:

cpython 3.14 @ ab2d84fe1023/Objects/unionobject.c ↗

This annotation covers runtime use of X | Y union types. See objects_uniontype_detail for union_new, __origin__, deduplication, and int | None creation.

Map

Lines	Symbol	Role
1-80	UnionType.__instancecheck__	`isinstance(x, int
81-180	UnionType.__subclasscheck__	`issubclass(bool, int
181-280	UnionType.__or__	`(int
281-400	__repr__ / __args__	Display and argument access

Reading

__instancecheck__

// CPython: Objects/unionobject.c:280 union_instancecheck
static PyObject *
union_instancecheck(PyObject *self, PyObject *instance)
{
    unionobject *alias = (unionobject *)self;
    Py_ssize_t nargs = PyTuple_GET_SIZE(alias->args);
    for (Py_ssize_t i = 0; i < nargs; i++) {
        PyObject *arg = PyTuple_GET_ITEM(alias->args, i);
        int res = PyObject_IsInstance(instance, arg);
        if (res < 0) return NULL;
        if (res) Py_RETURN_TRUE;
    }
    Py_RETURN_FALSE;
}

isinstance(x, int | str) is equivalent to isinstance(x, (int, str)). The check iterates over __args__. None in a union (int | None) is represented as type(None) in __args__, so isinstance(None, int | None) checks against NoneType.

UnionType.__or__

// CPython: Objects/unionobject.c:180 union_or
static PyObject *
union_or(PyObject *self, PyObject *other)
{
    /* Flatten: (int | str) | float -> int | str | float
       Not (int | str) | float (nested union) */
    PyObject *args = PyTuple_New(...);
    /* Copy args from self */
    unionobject *u = (unionobject *)self;
    for (int i = 0; i < PyTuple_GET_SIZE(u->args); i++) {
        PyTuple_SET_ITEM(args, i, Py_NewRef(PyTuple_GET_ITEM(u->args, i)));
    }
    /* Append other (or flatten if other is also a UnionType) */
    ...
    return make_union(args);
}

(int | str) | float produces int | str | float, not (int | str) | float. Unions are always flat. Deduplication is also applied: int | int produces int.

__repr__

// CPython: Objects/unionobject.c:380 union_repr
static PyObject *
union_repr(PyObject *self)
{
    unionobject *alias = (unionobject *)self;
    /* Join __args__ with " | " */
    PyObject *pieces = PyList_New(PyTuple_GET_SIZE(alias->args));
    for (int i = 0; i < PyTuple_GET_SIZE(alias->args); i++) {
        PyObject *arg = PyTuple_GET_ITEM(alias->args, i);
        PyObject *repr = _PyObject_GetAttrId(arg, &PyId___qualname__);
        PyList_SET_ITEM(pieces, i, repr);
    }
    return PyUnicode_Join(_Py_PIPE_SPACE, pieces);
}

repr(int | str | None) produces 'int | str | None'. NoneType is special-cased to display as None for readability.

gopy notes

UnionType.__instancecheck__ is objects.UnionTypeInstanceCheck in objects/union_type.go. UnionType.__or__ calls objects.MakeUnion which deduplicates using a set of type IDs. __repr__ joins the arg repr strings with " | ". The __args__ tuple is stored as objects.UnionType.Args []PyObject.