Lib/itertools.py / Modules/itertoolsmodule.c (part 5)

Source:

cpython 3.14 @ ab2d84fe1023/Modules/itertoolsmodule.c ↗

This annotation covers filtering and grouping itertools. See module_itertools4_detail for islice, takewhile, zip_longest, pairwise, and batched.

Map

Lines	Symbol	Role
1-80	chain.from_iterable	Flatten a lazy sequence of iterables
81-160	compress	Filter by a boolean selector iterable
161-240	dropwhile	Skip items while predicate is true
241-320	filterfalse	Yield items where predicate is false
321-500	groupby	Group consecutive equal (or key-equal) items

Reading

chain.from_iterable

// CPython: Modules/itertoolsmodule.c:420 chain_from_iterable
static PyObject *
chain_from_iterable(PyTypeObject *type, PyObject *it)
{
    /* chain.from_iterable([[1,2],[3,4]]) == chain([1,2],[3,4]) */
    chainobject *lz = (chainobject *)chain_new_internal(type, it);
    if (!lz) return NULL;
    lz->source = PyObject_GetIter(it);
    lz->active = NULL;  /* Will fetch first sub-iterable on next() */
    return (PyObject *)lz;
}

chain.from_iterable(it) is lazy: it does not advance it until the first next(). This differs from chain(*it) which evaluates it eagerly to unpack the arguments.

compress

// CPython: Modules/itertoolsmodule.c:680 compress_next
static PyObject *
compress_next(compressobject *lz)
{
    /* Yield data[i] when selectors[i] is truthy */
    while (1) {
        PyObject *data = (*Py_TYPE(lz->data)->tp_iternext)(lz->data);
        if (data == NULL) return NULL;
        PyObject *sel = (*Py_TYPE(lz->selectors)->tp_iternext)(lz->selectors);
        if (sel == NULL) { Py_DECREF(data); return NULL; }
        int ok = PyObject_IsTrue(sel);
        Py_DECREF(sel);
        if (ok > 0) return data;
        Py_DECREF(data);
    }
}

compress('ABCDE', [1,0,1,0,1]) yields A, C, E. Both iterables are advanced in lockstep; stops when the shorter one is exhausted.

groupby

// CPython: Modules/itertoolsmodule.c:1080 groupby_next
static PyObject *
groupby_next(groupbyobject *lz)
{
    /* Advance past items in the previous group */
    while (lz->currkey != NULL) {
        PyObject *newval = (*Py_TYPE(lz->it)->tp_iternext)(lz->it);
        if (newval == NULL) return NULL;
        Py_XDECREF(lz->currvalue);
        lz->currvalue = newval;
        PyObject *newkey = PyObject_CallOneArg(lz->keyfunc, newval);
        if (PyObject_RichCompareBool(newkey, lz->currkey, Py_EQ)) {
            Py_DECREF(newkey);
        } else {
            Py_SETREF(lz->currkey, newkey);
            break;
        }
    }
    /* Return (key, group_iterator) */
    ...
}

groupby only groups consecutive equal items. sorted first if you want all same-key items together. The group iterator shares the underlying iterator with the groupby object, so iterating the next group exhausts the previous one.

gopy notes

chain.from_iterable is module/itertools.ChainFromIterable in module/itertools/module.go. compress is module/itertools.Compress. groupby is module/itertools.GroupBy; it stores the current key and value, advancing the source iterator lazily when the outer iterator is advanced.