Objects/genericaliasobject.c

Source:

cpython 3.14 @ ab2d84fe1023/Objects/genericaliasobject.c ↗

Ga_Type is the type of objects produced by list[int], dict[str, int], and similar subscript operations on built-in types. It was introduced in Python 3.9 (PEP 585) to allow built-in collection types to serve directly as generic aliases without importing from typing.

Map

Lines	Symbol	Role
1-100	Ga_new	__class_getitem__ calls this to create a GenericAlias
101-300	__args__, __origin__, __parameters__	Introspection attributes
301-500	Ga_getitem	list[int][str] — further subscripting
501-700	Ga_call, Ga_instancecheck	list[int](...) and isinstance delegation

Reading

Creating a generic alias

// CPython: Objects/genericaliasobject.c:68 Ga_new
PyObject *
Py_GenericAlias(PyObject *origin, PyObject *args)
{
    if (!PyTuple_Check(args)) {
        args = PyTuple_Pack(1, args);
    }
    gaobject *alias = PyObject_GC_New(gaobject, &Ga_Type);
    alias->origin = Py_NewRef(origin);
    alias->args   = Py_NewRef(args);
    alias->parameters = NULL;  /* computed lazily */
    ...
    return (PyObject *)alias;
}

list[int] calls list.__class_getitem__(int), which calls Py_GenericAlias(list, int).

__parameters__

// CPython: Objects/genericaliasobject.c:180 ga_parameters
static PyObject *
ga_parameters(gaobject *self, void *unused)
{
    if (self->parameters == NULL) {
        /* Collect TypeVar instances from args recursively */
        self->parameters = _Py_make_parameters(self->args);
    }
    return Py_NewRef(self->parameters);
}

list[T].__parameters__ returns (T,). This is used by typing.get_args and type checkers.

__getitem__ (further subscripting)

// CPython: Objects/genericaliasobject.c:320 Ga_getitem
static PyObject *
Ga_getitem(gaobject *self, PyObject *args)
{
    /* list[T][int] -> list[int]: substitute TypeVars */
    PyObject *new_args = _Py_subs_parameters(self, self->args, self->parameters, args);
    return Py_GenericAlias(self->origin, new_args);
}

TypeVar substitution allows class MyList(list[T]): ... and MyList[int] to work.

gopy notes

GenericAlias is in objects/generic_alias.go. The __class_getitem__ slot on list, dict, tuple, set, and type must return a GenericAlias. isinstance(x, list[int]) delegates to list.__instancecheck__ (i.e., just checks isinstance(x, list), ignoring the type arg). TypeVar substitution is only needed for the typing module integration.