Add Items

append adds one item to the end

names = ["Ada"]
names.append("Grace")
names.append("Katherine")
print(names)

['Ada', 'Grace', 'Katherine']

append() changes the list and returns None. That is deliberate — it is a reminder that the work happened in place — and it produces a classic bug:

names = ["Ada"]
names = names.append("Grace")
print(names)

None

insert puts one item at a position

names = ["Ada", "Katherine"]
names.insert(1, "Grace")
print(names)

['Ada', 'Grace', 'Katherine']

The item lands at that index, pushing the rest along. An index past the end does not fail; it simply appends:

names = ["Ada"]
names.insert(99, "Grace")
print(names)

['Ada', 'Grace']

extend adds many items

extend() takes an iterable and adds each item separately:

names = ["Ada"]
names.extend(["Grace", "Katherine"])
print(names)

['Ada', 'Grace', 'Katherine']

Compare it with append(), which adds whatever you give it as a single item:

names = ["Ada"]
names.append(["Grace", "Katherine"])
print(names)
print(len(names))

['Ada', ['Grace', 'Katherine']]
2

The list now holds two items, and the second one is itself a list. That is sometimes what you want and usually not.

extend takes any iterable

Including a string, which is a sequence of characters — another surprise worth meeting on purpose:

letters = ["a"]
letters.extend("bc")
print(letters)

['a', 'b', 'c']

Joining with +

+ builds a new list rather than changing either one:

first = [1, 2]
second = [3, 4]
both = first + second
print(both)
print(first)

[1, 2, 3, 4]
[1, 2]

+= on a list behaves like extend(), changing it in place:

first = [1, 2]
first += [3]
print(first)

[1, 2, 3]

Which to use