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CurriculumReading Python Error Messages, for KidsName that exception

Common Python Errors: IndexError, KeyError, TypeError Explained

Recognise the common crash types by name, the way a symptom points to a diagnosis.

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Name that exception

You've met NameError. There are a few more crash TYPES worth recognising on sight — each one is a different diagnosis.

IndexError means: you asked a LIST for a position that doesn't exist.

hoard = ["gold", "gems"]
print(hoard[5])

Output

IndexError: list index out of range

KeyError means: you asked a DICT for a key that was never set.

dragon = {"name": "Ember"}
print(dragon["mana"])

Output

KeyError: 'mana'

TypeError means: you tried to combine two things that don't mix — most often, text and a number joined with +.

print("Level: " + 5)

Output

TypeError: can only concatenate str (not "int") to str

ValueError means: you tried to CONVERT text that was never a real number in the first place.

int('abc')

Output

ValueError: invalid literal for int() with base 10: 'abc'

ZeroDivisionError means exactly what it says: dividing by zero, which has no real answer.

print(10 / 0)

Output

ZeroDivisionError: division by zero

Five names, five different symptoms. Once you recognise the TYPE, you already know roughly where to look — before reading a single word of the message.

Check your understanding

party = ["Ember", "Blaze"]. print(party[9]) raises which error?

party = ["Ember", "Blaze"]
print(party[9])
  • IndexError — position 9 doesn't exist in a 2-item list
  • KeyError — 9 isn't a valid key
  • TypeError — 9 is the wrong type for indexing

Why: IndexError — party only has positions 0 and 1. Asking for position 9 is asking for something genuinely out of range, and IndexError is the specific name for exactly that.

score = int("high"). What error does this raise, and why?

score = int("high")
  • ValueError — 'high' isn't made of digits, so there's nothing to convert
  • TypeError — int() was given the wrong type
  • NameError — 'high' was never defined

Why: ValueError. int() happily accepts a string — that's normal — but "high" isn't digits, so there's nothing valid to convert into a number. ValueError specifically means "this value's CONTENT doesn't work," as opposed to TypeError's "this value's TYPE doesn't work."

What you'll practice

hoard = ["gold", "gems"] — only 2 items, at positions 0 and 1. This code asks for position 5, which doesn't exist. Run it, read the IndexError, then fix it to ask for a REAL position — the last item, gems.

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More from this topic

Name that exception is one lesson inside Reading Python Error Messages, for Kids — see the full lesson order and what the whole topic covers.

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