Is there a reason why something like `list[]` raises `SyntaxError` in Python?

Question

Let's say that I want to implement my custom list class, and I want to override __getitem__ so that the item parameter can be initialized with a default None, and behave accordingly:

class CustomList(list): def __init__(self, iterable, default_index): self.default_index = default_index super().__init__(iterable) def __getitem__(self, item=None): if item is None: item = self._default_index return super().__getitem__(item) iterable = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0] my_list = CustomList(iterable, 2)

This allows for my_list[None], but it would be awesome to have something like my_list[] inherently use the default argument.

Unfortunately that raises SyntaxError, so I'm assuming that the statement is illegal at the grammar level...my question is: why? Would it conflict with some other statements?

I'm very curious about this, so thanks a bunch to anyone willing to explain!

tdelaney · Accepted Answer · 2021-09-20 16:57:35Z

Its not syntactically useful. There isn't a useful way to programatically use my_list[] without literally hard-coding it as such. A single piece of code can't sometimes have a variable in the list reference and other times not. In that case, why not just have a different property that gets the default?

@property def default(self): return super().__getitem__(self.default) @property.setter def default(self, val): super().__setitem__(self.default, val)

object.__getitem__(self, val) is defined to have a required positional argument. Python is dynamic and so you can get away with changing that call signature, but that doesn't change how all the other code uses it.

All python operators have a magic method behind them and its always the case that the magic method could expose more features than the operator. Why not let + have a default? So, a = b + would be legal. Once again, that would not be syntactically useful - you could just expose a function if you want to do that.

Thank you very much for the explanation; it makes sense that they wouldn't implement it without a tangible benefit. Also, I found the addition of a dedicated property a good solution, so thanks for adding that too!

Possseidon · Accepted Answer · 2021-09-20 16:36:38Z

-1

__getitem__ always takes exactly one argument. You can kindof pass multiple arguments, but this actually just converts it into a tuple:

>>> a = [] >>> a[1, 2] Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: list indices must be integers or slices, not tuple

Note the "not tuple" in the error message.

answered Sep 20, 2021 at 16:36

Possseidon

5921 gold badge4 silver badges11 bronze badges

4 Comments

pho Over a year ago

This has nothing to do with why specifying no index is a syntax error

Possseidon Over a year ago

"always takes exactly one argument", i.e. no argument is not allowed. How does that not answer the question?

Max Shouman Over a year ago

@Possseidon actually I wasn't trying to mess with __getitem__ signature, but just to set a default value and use an 'empty' indexing notation, in a similar fashion as when you call a function with empty parentheses.

Possseidon Over a year ago

I meant __getitem__ as synonymous with indexing itself. Sorry if that wasn't clear. Indexing itself is what always takes exactly one parameter, which I tried to illustrate with the example.

Collectives™ on Stack Overflow

Is there a reason why something like `list[]` raises `SyntaxError` in Python?

2 Answers 2

1 Comment

4 Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

4 Comments

Related