Python IndexError: list index out of range

What this error means

Code asked a sequence — list, tuple, or string — for a position that does not exist. Python indexes from zero, so a list of n items accepts positive indexes 0 through n − 1. The single most important fact on this page: len(items) is a count, not a position. items[len(items)] is out of range on every list ever created.

Note what does not raise IndexError: slices. items[5:99] on a three-item list quietly returns what exists (nothing, here). Only single-position lookups crash — which is why an IndexError always points at an expression with one number inside the brackets.

A real example

colors = ["red", "green", "blue"]
print(colors[3])   # "the 3rd color"... but Python counts from 0

Traceback (most recent call last):
  File "app.py", line 2, in <module>
    print(colors[3])
          ~~~~~~^^^
IndexError: list index out of range

The third color as a human counts is colors[2]. The last item, always safely, is colors[-1].

Why it happens

• The length-as-index trap — items[len(items)] or a loop that runs to len(items) inclusively.
• Off-by-one loops — range(len(items) + 1), or a while-loop with <= where < belongs; works on every pass except the last.
• Human counting — asking for "item 3" with [3] when position 3 is the fourth element.
• The empty-list edge — items[0] crashes when a filter, a bad file, or a new user handed you zero elements.

How to debug it

1. Print len(items) and the failing index right before the crash line — the comparison is the diagnosis.
2. If the index comes from a loop, hand-run only the final iteration; boundary bugs live there.
3. If the index is a constant, decide whether it encodes human counting (subtract one) or "the last item" (use -1).
4. If the list can legitimately be empty, guard first: if items: — do not catch IndexError to paper over missing data.

Fix it interactively

Five free problems, each a different real-world shape of this exact error:

• Wrong index to get the last item easy
• A hard-coded index past the end easy
• A loop that reads one slot too many easy
• Nested list: which bracket failed? easy
• Off-by-one loop bounds easy

Related errors

• KeyError — the same missing-lookup story, for dictionaries instead of positions.
• ValueError — raised by list.index(x) when x is absent — a lookup by value, not position.

String slicing never raises but silently truncates — see off-by-one in string slicing and the full errors guide.