making : optional

[Skaperen]

what would be an example of a situation where code can be valid without : and valid with : and have different semantics? or can a language like Python that has optional : be possible?

if i leave out : i get a syntax error. is there a case where that does not happen?

[Gribouillis]

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>>> L = list(range(10)) >>> L[::-1] [9, 8, 7, 6, 5, 4, 3, 2, 1, 0] >>> L[:-1] [0, 1, 2, 3, 4, 5, 6, 7, 8] >>> L[-1] 9

[DeaD_EyE]

If there is a colon in the square brackets, it's an instance of slice.
A slice takes 3 arguments. [::] -> slice(None, None, None)

[::-1] -> slice(None, None, -1)
The : in the subscription is like a comma in the function call.
If you use a comma in the subscription, then the key is a tuple.


You can try it by yourself.
__class_getitem__ was introduced for typehints.
I used it because then I don't have to create an instance of the class.
It's like __getitem__ for instances.

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class GetItem:     def __class_getitem__(self, key):         if isinstance(key, slice):             print("Key is a slice")         elif isinstance(key, tuple):             print("Key is a tuple")             if all(isinstance(item, slice) for item in key):                 print("All items are slices")             elif any(isinstance(item, slice) for item in key):                 print("One or more items are a slice instance")         else:             print("Got something else")         return key                 print(GetItem[1]) print(GetItem[:]) print(GetItem[0:]) print(GetItem[1:3]) print(GetItem[:2]) print(GetItem[::-1]) print(GetItem[:,1,:])

[Skaperen]

i didn't think about slices although a different language could require tuples be inside () and use , in place of : in the square brackets. i was thinking about the : that follow keywords like def, if, else, for, while, try. except ... well, eventually follow. what if that language looked like:

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class GetItem     def __class_getitem__(self, key)         if isinstance(key, slice)             print("Key is a slice")         elif isinstance(key, tuple)             print("Key is a tuple")             if all(isinstance(item, slice) for item in key)                 print("All items are slices")             elif any(isinstance(item, slice) for item in key)                 print("One or more items are a slice instance")         else             print("Got something else")         return key ...