Why does str[64:] work while str[64] doesn't? [duplicate]

Question

>>> a = ["a", "b", "c", "d", "e", "f", "g", "h", "l"]
>>> a[30:]
[]
>>> a[:30]
['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'l']

I am trying to understand the logic behind this slicing. For example, when we try to reach element by indexing, it will give us an IndexError.

>>> a[12]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

We have got total 9 elements in the list. Yet, we can use any indexes which are greater than 8. Any technical explanations would be highly appreciated.

Answer 1

This makes sense if you see a slice like [i:j] as "make a list of all elements that have an index between i and j." If the original list has 3 elements, i is 5 and j is 7, then there are no elements that fulfill this requirement, so the result will be an empty list.

In practice, it turns out this property is useful quite often.