Rule request

Allow only a name, constant, and attribute in slice args.

Thesis

Correct:
array[3:7]
array[start_index:end_index]
array[index.start:index.end]

Wrong:
array[my_dict["start_index"]:my_list[-1]]
array[start_index():index.end()]

Should be:

start_index = my_dict["start_index"]
end_index = my_list[-1]
new_array = array[start_index:end_index]

start_index = start_index()
end_index = index.end()
new_array = array[start_index:end_index]

Reasoning

Complex grammar are difficult to read and should be stored in separate vars.

It's really difficult to review such many dependent constructions:
new_array = array[my_dict[":"]::get_step()]

It might also be worth forbidding an attribute, but I don't think it's a complex grammar.

class Slice:
    class MyArray:
        start = 1
        end = 1

new_array = array[Slice.MyArray.start:]

But not
new_array = array[Slice(arg_1).MyArray(arg_2, arg_3).start:]