An `AttributeError` is raised when code (or a unit test!) tries to access the attribute of an object but that object has no such attribute.

For example, a unit test expects a `Robot` object to have a `direction` attribute, but when it tried to access `robot.direction`, it does not exist.

This could also indicate a typo, such as using `"Hello".lowercase()` when the correct syntax is `"Hello".lower()`.

`"Hello".lowercase()` raises `AttributeError: 'str' object has no attribute 'lowercase'`.

Python raises an `IndexError` when an invalid index is used to look up a value in a sequence.

This often indicates the index is not computed properly and is often an off-by-one error.

A `ValueError` is usually raised when an invalid value is passed to function.

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Note, real square roots only exist for positive numbers.

Calling `math.sqrt(-1)` will raise `ValueError: math domain error` since `-1` is not a valid value for a square root.

In (mathematical) technical terms, -1 is not in the domain of square roots.