What does the slice mylist[:3] return?

The slice notation mylist[:3] is used to create a new list that includes elements from the beginning of mylist up to, but not including, the element at index 3. In Python, list indexing starts at 0, which means that the first element is at index 0, the second at index 1, the third at index 2, and the fourth at index 3.

When using the slice [:3], it begins from index 0 and captures the elements at indices 0, 1, and 2. Therefore, the resulting list will contain the first three elements of mylist. This behavior is fundamental to how list slicing works in Python, making it intuitive to select a sub-list of items based on their position in the original list.