What is the difference between append and extend on a Python list?

The short version

append adds whatever you give it as one single element, so appending a list produces a nested list. extend adds each item of an iterable one by one, flattening it into the existing list. Both modify the list in place and return None, which is a frequent source of bugs.

Seeing the difference

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

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

append treated [3, 4] as a single thing and tucked it in whole. extend walked through [3, 4] and added 3 and then 4.

When you want each

Use append to add one item, whatever its type, including when that item happens to be a list you want to keep whole. Use extend to concatenate the contents of another sequence onto the end.

rows = []
rows.append([1, 2])   # keep [1, 2] as one row
rows.append([3, 4])
# rows is [[1, 2], [3, 4]], a list of rows

The None trap

Because both methods change the list in place and return None, assigning the result throws the list away.

a = [1, 2]
a = a.append(3)   # BUG: a is now None

The fix is to call the method for its effect and not reassign.

a = [1, 2]
a.append(3)   # a is now [1, 2, 3]

extend accepts any iterable

extend does not need a list. It accepts anything you can loop over, including a string, which extends by characters and often surprises people.

c = [1, 2]
c.extend("ab")
print(c)   # [1, 2, 'a', 'b']

The plus operators

For comparison, a + b creates a new list and leaves the originals alone, while a += b behaves like extend and modifies a in place. Use + when you want a fresh list and want to keep the originals untouched.