让我们disassemble {}:

>>> from dis import dis
>>> dis(lambda: {})
  1           0 BUILD_MAP                0
              3 RETURN_VALUE

Python 2.7 implementation of BUILD_MAP

TARGET(BUILD_MAP)
{
    x = _PyDict_NewPresized((Py_ssize_t)oparg);
    PUSH(x);
    if (x != NULL) DISPATCH();
    break;
}

Python 3.5 implementation of BUILD_MAP

TARGET(BUILD_MAP) {
    int i;
    PyObject *map = _PyDict_NewPresized((Py_ssize_t)oparg);
    if (map == NULL)
        goto error;
    for (i = oparg; i > 0; i--) {
        int err;
        PyObject *key = PEEK(2*i);
        PyObject *value = PEEK(2*i - 1);
        err = PyDict_SetItem(map, key, value);
        if (err != 0) {
            Py_DECREF(map);
            goto error;
        }
    }

    while (oparg--) {
        Py_DECREF(POP());
        Py_DECREF(POP());
    }
    PUSH(map);
    DISPATCH();
}

这是更多的代码.

EDIT:

Python 3.4实现的BUILD_MAP id与2.7完全相同(谢谢@user2357112).我挖得更深，看起来Python 3分钟的dict大小是8PyDict_MINSIZE_COMBINED const

PyDict_MINSIZE_COMBINED是任何新的非拆分dict的起始大小.8允许dict的活动条目不超过5个；实验表明，这对于大多数dict(主要由通常为传递关键字参数而创建的小dict组成)来说已经足够了.将其设为8，而不是4，可以减少大多数词典的大小调整次数，而不会占用大量额外内存.

看看_PyDict_NewPresized in Python 3.4

PyObject *
_PyDict_NewPresized(Py_ssize_t minused)
{
    Py_ssize_t newsize;
    PyDictKeysObject *new_keys;
    for (newsize = PyDict_MINSIZE_COMBINED;
         newsize <= minused && newsize > 0;
         newsize <<= 1)
        ;
    new_keys = new_keys_object(newsize);
    if (new_keys == NULL)
        return NULL;
    return new_dict(new_keys, NULL);
}

2.7年后

PyObject *
_PyDict_NewPresized(Py_ssize_t minused)
{
    PyObject *op = PyDict_New();

    if (minused>5 && op != NULL && dictresize((PyDictObject *)op, minused) == -1) {
        Py_DECREF(op);
        return NULL;
    }
    return op;
}

在这两种情况下，minused的值均为1.

Python2.7创建一个空的dict，Python3.4创建一个7元素的dict.