Python index(): Find a Value and Handle a Missing Match

Start with a small, explicit task

A fictional route visits Central twice. Find the first visit, search beyond it for the return visit, and inspect what happens when a bounded search excludes that return. This exercise uses label equality, not geographical distance or a unique stop identifier.

Run the complete example

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

stops = ["Depot", "Central", "Park", "Central", "Harbor"]
first = stops.index("Central")
second = stops.index("Central", first + 1)
try:
    stops.index("Central", 2, 3)
except ValueError:
    bounded = "not found"
else:
    raise AssertionError("expected no Central in that slice")
assert first == 1 and second == 3
assert bounded == "not found"
assert stops[second] == "Central"
print(first, second)
print(bounded)

Explain the result

The first match is at zero-based position 1. Starting the next search at 2 skips that first match and finds position 3. It returns 3, not 1, because start changes where the search begins rather than renumbering the list. The final lookup checks that this index still selects Central in the original sequence.

The bounded search covers position 2 but excludes its stop bound 3. Park is the only examined item, so ValueError is the expected result. The except branch deliberately handles this specific missing-match case. It does not suppress unrelated failures or assign a fake valid index.

Avoid these mistakes

Do not assume list.index behaves like a string find method that returns -1. An accidental -1 used as a list index selects the last item and can hide a missing-match bug. For many repeated searches, consider whether you need a lookup structure, but account for duplicate labels before replacing the sequence.