Objects/iterobject.c

cpython 3.14 @ ab2d84fe1023/Objects/iterobject.c ↗

Objects/iterobject.c defines the two general-purpose iterators created by the iter() built-in. Most built-in types have their own dedicated iterators; these are the fallbacks for objects that do not implement __iter__ but do implement __getitem__, and for the two-argument form iter(callable, sentinel).

Map

Lines	Symbol	Role
1-130	seqiterobject, iter_new, iter_iternext	Index-based iterator over __getitem__
131-230	calliterobject, calliter_new, calliter_iternext	Callable-plus-sentinel iterator

Reading

seqiterobject

When iter(obj) is called on an object with __getitem__ but no __iter__, CPython creates a seqiterobject. Each __next__ call does obj[it_index] and increments the index. When IndexError or StopIteration is raised, the iterator is exhausted.

// CPython: Objects/iterobject.c:62 iter_iternext
static PyObject *
iter_iternext(PyObject *iterator)
{
    seqiterobject *it = (seqiterobject *)iterator;
    PyObject *seq = it->it_seq;
    if (seq == NULL) return NULL;
    PyObject *result = PySequence_GetItem(seq, it->it_index++);
    if (result != NULL) return result;
    if (PyErr_ExceptionMatches(PyExc_IndexError) ||
        PyErr_ExceptionMatches(PyExc_StopIteration)) {
        PyErr_Clear();
        it->it_seq = NULL;
        Py_DECREF(seq);
    }
    return NULL;
}

calliterobject

iter(callable, sentinel) creates a calliterobject. Each __next__ calls callable() with no arguments. If the result equals sentinel (by ==), the iterator raises StopIteration.

// CPython: Objects/iterobject.c:185 calliter_iternext
static PyObject *
calliter_iternext(calliterobject *it)
{
    PyObject *result = _PyObject_CallNoArgs(it->it_callable);
    if (result != NULL) {
        int ok = PyObject_RichCompareBool(it->it_sentinel, result, Py_EQ);
        if (ok == 0) return result;
        ...
        return NULL;
    }
    ...
}

gopy notes

Both iterator types are used by the FOR_ITER opcode path in vm/eval_simple.go. The seqiterobject pattern is also used by tuple and list when their dedicated iterators are not available. Both will be ported as part of the core objects layer.