Common errors, explained

IndexError: list index out of range

You asked for an index the list does not have. Almost always a loop bound that is one too far, or an empty list you assumed had items.

IndexError: string index out of range

Same cause as the list version: you asked for a character position the string does not have.

AttributeError: 'NoneType' object has no attribute

Something returned None and you called a method on it. Usually a lookup that failed, or a function that forgot to return.

TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'

You did arithmetic with a None. Something in the sum was missing and nobody checked.

UnboundLocalError: cannot access local variable

You assigned to a name somewhere inside a function, which made it local for the whole function — including before the assignment runs.

RecursionError: maximum recursion depth exceeded

A function kept calling itself without reaching a stopping condition.

list.sort() returns None

sort() sorts in place and gives back None. Assigning its result throws the list away.

TypeError: '>' not supported between instances of 'str' and 'int'

You compared text with a number. The value arrived as a string and was never converted.

TypeError: list indices must be integers or slices, not float

You used a float as an index. In Python 3, / always produces a float — even when the result looks whole.

TypeError: Cannot read properties of undefined

You read a property off something that does not exist. The crash is usually far from the real mistake — the value went missing earlier.

ReferenceError: Cannot access before initialization

You used a let or const before its declaration line executed. This is the temporal dead zone.

[object Promise] in output

A promise was used where its resolved value was expected — nearly always a missing await.