Python/ceval.c (part 24)

Source:

cpython 3.14 @ ab2d84fe1023/Python/ceval.c ↗

This annotation covers collection construction opcodes. See python_ceval23_detail for exception and context manager opcodes.

Map

Lines	Symbol	Role
1-80	BUILD_MAP	Create a dict from key-value pairs on the stack
81-160	BUILD_SET	Create a set from items on the stack
161-260	DICT_MERGE	{**a, **b} — merge two dicts into a new one
261-360	LIST_EXTEND	[*a, *b] — extend a list with an iterable
361-400	SET_UPDATE	{*a, *b} — update a set with an iterable

Reading

BUILD_MAP

// CPython: Python/ceval.c:4280 BUILD_MAP
inst(BUILD_MAP, (values[oparg*2] -- map)) {
    map = _PyDict_FromItems(
        &values[0], 2,
        &values[1], 2,
        oparg);
    if (map == NULL) ERROR_IF(true, error);
}

{'a': 1, 'b': 2} pushes keys and values alternately, then BUILD_MAP 2 pops them into a dict. _PyDict_FromItems takes strided pointers: keys[0], keys[2], ... and values[1], values[3], .... Duplicate keys are silently overwritten (last one wins).

DICT_MERGE

// CPython: Python/ceval.c:4380 DICT_MERGE
inst(DICT_MERGE, (update -- )) {
    /* TOS is the dict to merge into (a `BUILD_MAP` result).
       oparg points to the function name for error messages. */
    PyObject *dict = PEEK(2);
    if (_PyDict_MergeEx(dict, update, 2) < 0) {
        /* _PyDict_MergeEx with flag=2 raises TypeError on duplicate keys */
        format_kwargs_error(tstate, PEEK(3 + oparg), update);
        ERROR_IF(true, error);
    }
}

{**a, **b} compiles to BUILD_MAP 0, then DICT_MERGE a, DICT_MERGE b. _PyDict_MergeEx(..., 2) raises TypeError if both dicts share a key (unlike dict.update which silently overwrites).

LIST_EXTEND

// CPython: Python/ceval.c:4460 LIST_EXTEND
inst(LIST_EXTEND, (iterable --)) {
    PyObject *list = PEEK(1 + oparg);
    int err = _PyList_Extend((PyListObject *)list, iterable);
    if (err < 0) {
        /* Check for non-iterable to give better error message */
        if (PyErr_ExceptionMatches(PyExc_TypeError) &&
            !(Py_TYPE(iterable)->tp_iter || PySequence_Check(iterable))) {
            ...
        }
        ERROR_IF(true, error);
    }
}

[1, *a, 2, *b] compiles to BUILD_LIST 1 (for the 1), then LIST_EXTEND for a, LOAD_CONST 2, LIST_APPEND, LIST_EXTEND for b. _PyList_Extend calls PySequence_Fast to get items in bulk.

SET_UPDATE

// CPython: Python/ceval.c:4520 SET_UPDATE
inst(SET_UPDATE, (iterable --)) {
    PyObject *set = PEEK(1 + oparg);
    int err = PySet_Update(set, iterable);
    ERROR_IF(err < 0, error);
}

{1, *a, 2, *b} builds a set and extends it with each splat. PySet_Update adds all elements of the iterable; duplicates are silently ignored.

gopy notes

BUILD_MAP is vm.BuildMap in vm/eval_gen.go. It calls objects.DictFromItems. DICT_MERGE calls objects.DictMergeEx(..., 2) which raises on duplicate keys. LIST_EXTEND calls objects.ListExtend. SET_UPDATE calls objects.SetUpdate.