Change what the *splat and **splatty-splat operato

Question

How do you override the result of unpacking syntax *obj and **obj?

For example, can you somehow create an object thing which behaves like this:

>>> [*thing]
['a', 'b', 'c']
>>> [x for x in thing]
['d', 'e', 'f']
>>> {**thing}
{'hello world': 'I am a potato!!'}

Note: the iteration via __iter__ ("for x in thing") returns different elements from the *splat unpack.

I had a look inoperator.mul and operator.pow, but those functions only concern usages with two operands, like a*b and a**b, and seem unrelated to splat operations.

Answer 1

* iterates over an object and uses its elements as arguments. ** iterates over an object's keys and uses __getitem__ (equivalent to bracket notation) to fetch key-value pairs. To customize * and **, simply make your object an iterable or mapping:

class MyIterable(object):
    def __iter__(self):
        return iter([1, 2, 3])

class MyMapping(collections.Mapping):
    def __iter__(self):
        return iter('123')
    def __getitem__(self, item):
        return int(item)
    def __len__(self):
        return 3

If you want * and ** to do something besides what's described above, you can't. I don't have a documentation reference for that statement (since it's easier to find documentation for "you can do this" than "you can't do this"), but I have a source quote. The bytecode interpreter loop in PyEval_EvalFrameEx calls ext_do_call to implement function calls with * or ** arguments. ext_do_call contains the following code:

        if (!PyDict_Check(kwdict)) {
            PyObject *d;
            d = PyDict_New();
            if (d == NULL)
                goto ext_call_fail;
            if (PyDict_Update(d, kwdict) != 0) {

which, if the ** argument is not a dict, creates a dict and performs an ordinary update to initialize it from the keyword arguments (except that PyDict_Update won't accept a list of key-value pairs). Thus, you can't customize ** separately from implementing the mapping protocol.

Similarly, for * arguments, ext_do_call performs

        if (!PyTuple_Check(stararg)) {
            PyObject *t = NULL;
            t = PySequence_Tuple(stararg);

which is equivalent to tuple(args). Thus, you can't customize * separately from ordinary iteration.

It'd be horribly confusing if f(*thing) and f(*iter(thing)) did different things. In any case, * and ** are part of the function call syntax, not separate operators, so customizing them (if possible) would be the callable's job, not the argument's. I suppose there could be use cases for allowing the callable to customize them, perhaps to pass dict subclasses like defaultdict through...

Answer 2

I did succeed in making an object that behaves how I described in my question, but I really had to cheat. So just posting this here for fun, really -

class Thing:
    def __init__(self):
        self.mode = 'abc'
    def __iter__(self):
        if self.mode == 'abc':
            yield 'a'
            yield 'b'
            yield 'c'
            self.mode = 'def'
        else:
            yield 'd'
            yield 'e'
            yield 'f'
            self.mode = 'abc'
    def __getitem__(self, item):
        return 'I am a potato!!'
    def keys(self):
        return ['hello world']

The iterator protocol is satisfied by a generator object returned from __iter__ (note that a Thing() instance itself is not an iterator, though it is iterable). The mapping protocol is satisfied by the presence of keys() and __getitem__. Yet, in case it wasn't already obvious, you can't call *thing twice in a row and have it unpack a,b,c twice in a row - so it's not really overriding splat like it pretends to be doing.