append vs extend vs + for lists

By Pradyumna Chippigiri

February 12, 2026

When combining lists in Python, append is not the same as extend or +.

append(x)

It adds the entire x as one single element to the list , and it can create nesting if x is a list or a tuple.

feed = []
x = [(1,"a"), (2,"b")]
feed.append(x)
# feed = [[(1,"a"), (2,"b")]] 

extend(iterable)

Takes all elements from another list and adds them into our list (in-place).. meaning doesnt create a new list, but modifies the existing list.

feed = []
x = [(1,"a"), (2,"b")]
feed.extend(x)
# feed = [(1,"a"), (2,"b")]     

+

Concatenates the new list and returns a new list.

feed = [(0, "Z")]
feed = feed + [(1, "a"), (2, "b")]
# feed = [(0, "Z"), (1, "a"), (2, "b")]    # new list feed is created 

• Use append when we want to add one-one items.
• Use extend (or +=) when we want to add many items from another list.
• Use + when we want a new combined list.

Why it matters (sorting example)

If our list should be a flat list of tuples like [(time, id), ...], you want:

feed.extend(self.tweets[f])   # or feed += self.tweets[f]
feed.sort(key=lambda x: x[0], reverse=True)

If we accidentally do append, we'll get a list-of-lists like this :

feed.append(self.tweets[f])
# feed = [[(time, id), ...], [(time, id), ...], ...]

and sorting becomes annoying.

This design twitter question , had some edge cases which helped me understand this.