Lists and changing one item

The idea

A list stores several values in order inside square brackets. Commas separate the items: labels = ["read", "code"]. The first position is zero, so labels[0] is "read". len(labels) counts items. For a nonempty list, labels[-1] reads the last item; an empty list has no valid item to read.

Lists are mutable. labels[0] = "review" replaces one item, while labels.append("test") adds an item at the end. append changes the list and returns None; it does not return the updated list. Keep the original variable and call the method on its own statement.

A list is a useful input for a loop or function. First practice tracing a small list manually: write its positions, change one item, then append another. Later chapters explain sets, dictionaries, copying, and shared references. You can learn the basic container before those extra rules.

labels = ["read", "code"]
labels[0] = "review"
labels.append("test")
print(labels[0])
print(len(labels))
print(labels[-1])

review
3
test

Replacing index zero keeps two items. Appending adds a third, which can be read with the negative index -1.