Python Substring: Slice Bounds, Negative Indexes and Steps

Start with a small, explicit task

Inspect the fictional label PK-7348-X. Extract its numeric portion, its prefix and final marker, then reverse it and sample every other character. Check a slice beyond the end alongside a single invalid index to see how their failure behavior differs.

Run the complete example

This original example was checked with Python 3.12.14. Save it as python_substring.py and run it with Python 3.12. It uses only the standard library. The assertions check the stated results.

label = "PK-7348-X"
number = label[3:7]
assert number == "7348"
assert label[:2] == "PK"
assert label[-1:] == "X"
assert label[::2] == "P-38X"
assert label[::-1] == "X-8437-KP"
assert label[99:] == ""
assert "7348" in label
try:
    label[99]
except IndexError:
    result = "single index rejected"
else:
    raise AssertionError("expected invalid index")
print(number, label[-1:])
print(label[::2])
print(label[::-1])
print(result)

Explain the result

Positions 3 through 6 contain the four digits; the stop bound 7 is excluded. The omitted start in [:2] begins at the left edge. The negative start in [-1:] selects the final character through the end. A step of two samples alternating positions, and a negative step reverses direction.

The far-right slice returns an empty string, while asking directly for character 99 raises IndexError. This distinction makes slicing convenient for partial extraction, but it can also conceal an invalid expected format. If the label must contain exactly four digits at specific positions, validate that contract rather than assuming a non-crashing slice proves it.

Avoid these mistakes

Keep string indexing distinct from user-perceived characters in general Unicode text: this ASCII label avoids composed-character questions. Do not confuse substring extraction with searching. A slice needs known positions; membership answers only whether a fragment occurs. A zero step is invalid and is not a way to request an empty result.