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IndexError: list index out of range

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

What Python is actually saying

You asked for position 3. There is no position 3.

Here’s the rule that causes nearly all of these: Python counts from zero.

Python
shelf = ["cleanser", "toner", "serum"]

Position012
Itemcleansertonerserum

Three items, and the last one sits at position 2, not 3. So a list of three has valid positions 0, 1 and 2 — and shelf[3] is one step past the end.

The rule: a list of N items has valid positions 0 to N−1.

Cause 1: off by one

Python
shelf[3]          # there is no 3
shelf[2]          # "serum" — the last one

Or count backwards, which never goes wrong:

Python
shelf[-1]         # "serum" — the last item, whatever the length
shelf[-2]         # "toner" — second from the end

[-1] is worth the habit. It’s always the last item, and you never have to know how many there are.

Cause 2: looping one step too far

Python
for i in range(1, len(shelf) + 1):
    print(shelf[i])          # IndexError on the last pass

range() already stops one before the number you give it, which is exactly what a zero-counted list needs. So the plain version is correct:

Python
for i in range(len(shelf)):
    print(shelf[i])

Better still, skip the counting entirely:

Python
for product in shelf:
    print(product)

You cannot get an IndexError this way, because you never name a position.

Cause 3: the list is empty

Python
shelf = []
print(shelf[0])          # IndexError — there's no 0 either

An empty list has no valid positions at all. Check before you reach in:

Python
if shelf:
    print(shelf[0])
else:
    print("Nothing on the shelf yet")

if shelf: reads as if there is anything in the shelf. It’s one of the most useful safety checks in Python.

What to check, in order

  1. How many items does the list actually have? Print len(shelf)
  2. Highest valid position is that number minus one
  3. Could the list be empty at this point?
  4. Can you loop over the items directly instead of by position?

Remember this

Zero-based counting isn’t Python being awkward. It’s consistent everywhere in the language, and [-1] means you rarely have to count at all.

Every error a beginner meets, explained calmly, in the Python Troubleshooting Handbook.

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