Objects/genericaliasobject.c (part 2)

Source:

cpython 3.14 @ ab2d84fe1023/Objects/genericaliasobject.c ↗

This annotation covers runtime type checking and nested parameterization. See objects_genericalias_detail for Ga_new, __origin__, __args__, and list[int] creation.

Map

Lines	Symbol	Role
1-80	GenericAlias.__getitem__	Further parameterize: dict[str, list][int]
81-180	__instancecheck__	isinstance(x, list[int]) — check origin type only
181-280	__subclasscheck__	issubclass(list, list[int]) — check origin
281-400	__mro_entries__ / __repr__	Class body resolution and display

Reading

GenericAlias.__getitem__

// CPython: Objects/genericaliasobject.c:380 ga_getitem
static PyObject *
ga_getitem(PyObject *self, PyObject *args)
{
    gaobject *alias = (gaobject *)self;
    /* Substitute type variables: list[T][int] -> list[int] */
    PyObject *new_args = _Py_subs_parameters(self, alias->args, alias->parameters, args);
    return Py_GenericAlias(alias->origin, new_args);
}

list[T][int] works when T is a TypeVar. _Py_subs_parameters replaces each TypeVar in alias->args with the corresponding concrete type from args. If no TypeVars are present, subscripting a GenericAlias raises TypeError.

__instancecheck__

// CPython: Objects/genericaliasobject.c:480 ga_instancecheck
static PyObject *
ga_instancecheck(PyObject *self, PyObject *instance)
{
    /* isinstance(x, list[int]) is equivalent to isinstance(x, list).
       The type argument is NOT checked at runtime. */
    gaobject *alias = (gaobject *)self;
    int r = PyObject_IsInstance(instance, alias->origin);
    return PyBool_FromLong(r);
}

isinstance([], list[int]) returns True for any list, regardless of element types. Python's generics are erased at runtime; type arguments exist only for static type checkers. A DeprecationWarning is raised to discourage this usage pattern.

__mro_entries__

// CPython: Objects/genericaliasobject.c:540 ga_mro_entries
static PyObject *
ga_mro_entries(PyObject *self, PyObject *bases)
{
    /* When used as a base class: class MyList(list[int]): ...
       Return the origin type so the MRO uses list, not list[int]. */
    gaobject *alias = (gaobject *)self;
    return PyTuple_Pack(1, alias->origin);
}

class MyList(list[int]): pass is valid. __mro_entries__ returns (list,) so the actual MRO uses list as the base. The type argument is recorded in __orig_bases__ for introspection by typing.get_type_hints.

__repr__

// CPython: Objects/genericaliasobject.c:280 ga_repr
static PyObject *
ga_repr(PyObject *self)
{
    gaobject *alias = (gaobject *)self;
    /* Produce "list[int]" or "dict[str, list[int]]" */
    PyObject *origin_repr = _PyObject_GetAttrId(alias->origin, &PyId___qualname__);
    PyObject *args_repr = ... /* join with ", " */;
    return PyUnicode_FromFormat("%U[%U]", origin_repr, args_repr);
}

repr(list[int]) produces 'list[int]'. For nested aliases like dict[str, list[int]], the inner alias repr is called recursively. __qualname__ is used instead of __name__ to correctly display nested classes.

gopy notes

GenericAlias.__getitem__ is objects.GenericAliasGetItem in objects/generic_alias.go. __instancecheck__ delegates to objects.IsInstance(instance, alias.Origin). __mro_entries__ returns a single-element tuple containing the origin type. __repr__ uses objects.Repr recursively on args.