KeyError and IndexError (Python)

In short: A key is missing from the dictionary (KeyError) or a list index lies outside the list (IndexError).

In more detail: Both errors arise when accessing elements that do not exist. They are cousins of Java’s ArrayIndexOutOfBoundsException.

In Depth

KeyError

age = {"Mia": 17}
print(age["Tom"])   # KeyError: 'Tom'

The key "Tom" is not in the dictionary. Typos and capitalisation count too. Solutions:

• age.get("Tom") returns None, age.get("Tom", 0) a default value.
• Check beforehand: if "Tom" in age:.
• Use collections.defaultdict or setdefault.
• With JSON data print the actual keys (print(data.keys())).

IndexError

numbers = [10, 20, 30]
print(numbers[3])   # IndexError: list index out of range

Valid indices are 0 to len(list) - 1. Frequent: off-by-one in loops (range(len(x)+1)), empty lists, wrong index after deleting elements. Solutions:

• Check if i < len(numbers): or if numbers: for “not empty”.
• Iterate with for n in numbers: instead of indices.
• Negative indices count from the end: numbers[-1].
• Use try/except IndexError only where absence is expectable.

Troubleshooting

Read the line number in the traceback, print len() or the keys before that line and check whether the data really looks as you think (empty response, different structure, None). The debugger or print at the right place saves a lot of time.

See also: ArrayIndexOutOfBoundsException, TypeError, ModuleNotFoundError