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This is a toy example that exposes the problem, but the problem itself is not a toy problem. The key point is that calculating slice indices cause executing Python code and releases GIL. In multithread program a sequence can be changed not in toy __index__ method, but in other thread, in legitimate code. This is very hardly reproducible bug.
Variants B are not efficient. To determine the size of a sequence we should call its __len__() method. This is less efficient than using macros Py_SIZE() or PyUnicode_GET_LENGTH(). And it is not always possible to pass a sequence. In multidimensional array there is no such sequence (see for example _testbuffer.ndarray).