Why Does Out-of-Range Substring Slicing in Python Return an Empty String Instead of an Error?

Out-of-Range Substring Slicing: Why It Works

While substring slicing in Python typically results in an error when the index is out of range, there's an exception to this behavior. Consider the operation 'example'[999:9999]: instead of an error, it returns an empty string.

This is because substring slicing with out-of-range indices doesn't cause an exception. Instead, Python interprets it as a range that extends beyond the sequence's length, resulting in an empty subsequence. This behavior differs from indexing a single element, which will yield an error if the index is out of range.

The distinction between indexing and slicing becomes apparent when dealing with strings and lists. For example, when indexing a list, an out-of-range index will return an error, while slicing will result in an empty list:

[0, 1, 2, 3, 4, 5][3]  # Returns 3
[0, 1, 2, 3, 4, 5][3:4]  # Returns [3]

In contrast, strings act differently. Since a single character is intrinsically a 1-character string in Python, indexing a string with an out-of-range index effectively yields an empty string:

'example'[3]  # Returns 'm' (the character at index 3)
'example'[3:4]  # Also returns 'm' (a substring of length 1)

In summary, substring slicing with out-of-range indices does not cause an error, as it's interpreted as a range that extends beyond the sequence's length, resulting in an empty subsequence. Indexing a single element, however, will trigger an error if the index is out of range.

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