Use append() to add one item at the end of a list, extend() to add the items from an iterable at the end, and insert() to place one item at a particular position. All three modify the existing list and return None.
How append(), extend() and insert() differ
| Method | What you pass | What it does | Where it adds items | Return value |
|---|---|---|---|---|
append(value) |
One value | Adds that value as one list item | At the end | None |
extend(iterable) |
An iterable | Adds each item yielded by the iterable | At the end | None |
insert(index, value) |
An index and one value | Adds the value before the item at that index | At the specified position | None |
These are methods for changing a list in place, not expressions that produce a new list. The Python 3.14.8 data structures tutorial defines their behavior and return values.
Use append() to add one item
append(value) puts its argument at the end of the list as a single element. If that value is itself a list or another collection, append() keeps it nested rather than unpacking it.
items = ["a", "b"]
items.append(["c", "d"])
print(items)
# ['a', 'b', ['c', 'd']]Use append() when the entire object should become one entry—for example, when adding a record, tuple, or sublist as a single element.
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Use extend() to add items from an iterable
extend(iterable) reads items from its argument and adds them individually to the end of the list. The argument can be a list, tuple, string, or another iterable.
items = ["a", "b"]
items.extend(["c", "d"])
print(items)
# ['a', 'b', 'c', 'd']A string is iterable, so extending with a string adds its characters one at a time. Appending the same string adds it as one element instead.
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items = ["a", "b"]
items.append("cat")
print(items)
# ['a', 'b', 'cat']
items = ["a", "b"]
items.extend("cat")
print(items)
# ['a', 'b', 'c', 'a', 't']The Python 3.14.8 built-in types reference describes lists as mutable sequences and gives strings and tuples as examples of iterables used to construct lists.
Use insert() to add one item at a position
insert(index, value) places one value before the element at the given index. Index 0 means the front of the list.
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items = ["a", "b"]
items.insert(1, "x")
print(items)
# ['a', 'x', 'b']Inserting at the current length puts the value at the end, the same position where append() adds it:
items = ["a", "b"]
items.insert(len(items), "c")
print(items)
# ['a', 'b', 'c']All three methods change the original list
Because these methods mutate the list, call them on the list itself and do not assign their result back to the variable. Each method returns None, so this mistake replaces items with None:
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items = ["a", "b"]
items = items.append("c") # items is now NoneInstead, call the method on its own line:
items = ["a", "b"]
items.append("c")Choose the method that matches the job
- Adding one object as a single final element: use
append(value). - Adding every item from a collection or other iterable at the end: use
extend(iterable). - Adding one item before a particular position: use
insert(index, value).
Consider performance for repeated front insertions
Appending at the end is fast. Inserting at the beginning is slower because the existing elements have to shift to make room. For a first-in, first-out queue with frequent operations at both ends, Python’s tutorial recommends collections.deque rather than repeatedly using insert(0, item). Lists remain suitable for stack-like use, where items are added and removed at the end.
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