Could dict.__init__() return kwargs if no args?

Core Development
1

As title. More specifically, could dict_init in dictobject.c return safely kwds if this is not NULL, and if args is NULL or empty? If so, maybe this will speedup keyword-only dict construction.

2

dict.__init__() does not return a dict object. It updates already created dict object. What exactly change do you propose?

3

You’re right, I confused with __new__. I was thinking about something like this:

static PyObject *
dict_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    PyObject *self;
    PyDictObject *d;
    PyDictObject *kwdict = (PyDictObject *)kwds;
    
    const int kwds_size = ((kwds != NULL) 
        ? kwdict->ma_used 
        : 0
    );
    
    PyObject* arg = NULL;
    
    if (! PyArg_UnpackTuple(args, __func__, 0, 1, &arg)) {
        return NULL;
    }
    
    const int arg_size = ((arg != NULL) 
        ? PyObject_Length(arg) 
        : 0
    );
    
    if (kwds_size && ! arg_size) {
        return kwdict;
    }
    
    [...]
}

static int
dict_init(PyObject *self, PyObject *args, PyObject *kwds)
{
    if (self == kwds) {
        return 0;
    }
    
    return dict_update_common(self, args, kwds, "dict");
}
4

It will cause the behavior change.

kwargs = {'a': 1}
d = dict(**kwargs)
assert d['a'] == 1
kwargs['a'] = 2
assert d['a'] == 2
5

I see. I thought that the dictionary passed with the “star-star” operator was copied. Thank you for the explaining.