Why Doesn't Python Raise an Error for Out-of-Range Substring Slicing?

Substring Slicing with Index Out of Range: Duality and Empty Sequences

In Python, accessing elements of a sequence using the slicing operator, such as 'example'[999:9999], can lead to unexpected behavior. Unlike indexing individual elements using 'example'[9], which raises an error, slicing outside the bounds of a sequence does not.

This behavior stems from the fundamental difference between indexing and slicing. Indexing a sequence, such as 'example'[3], returns a single item. However, slicing a sequence, such as 'example'[3:4], returns a subsequence of items.

When indexing an element that does not exist, such as 'example'[9], there is no item to return, hence the error. In contrast, when slicing a sequence outside bounds, an empty sequence can be returned. This is because a slice of a sequence from an index beyond its length to an index beyond its length or up to the end of the sequence is an empty sequence.

To illustrate this further, consider the following behavior with lists:

>>> [0, 1, 2, 3, 4, 5][3]
3
>>> [0, 1, 2, 3, 4, 5][3:4]
[3]

In this case, the difference between indexing and slicing is evident. With strings, the results appear identical because there is no concept of an individual character in Python outside of a string. Rather, a single character is a 1-character string.

Therefore, substring slicing with an index out of range in Python does not result in an error because there is a meaningful result: an empty sequence. This behavior allows for flexible and concise coding when handling sequences of various lengths.

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