This article shows the difference between append() and extend() for Python Lists.

append(x)

Appends x to the end of the sequence. This means it inserts one item. In the following example we have a nested list as the third item as a consequence.

a = [1, 2]
b = [3, 4]

a.append(b)
# a = [1, 2, [3, 4]]

extend(iterable)

Extends the list by appending elements from the iterable.

a = [1, 2]
b = [3, 4]

a.extend(b)
# a = [1, 2, 3, 4]

For example, b could also be a tuple. This works and the resulting list would be the same. It's worth mentioning that the += operator does the same as .extend().

a = [1, 2]
b = (3, 4)

a += b
# a = [1, 2, 3, 4]

On the other hand, the single + operator with a new assignement is not allowed for different types:

a = [1, 2]
b = (3, 4)

a = a + b
# TypeError: can only concatenate list (not "tuple") to list