After this lesson, you will be able to:

• Add one item with append() or several items with extend().
• Insert an item at a chosen position without replacing an existing one.
• Remove the first matching value with remove().
• Remove and retrieve an item with pop(), using its default or a specified index.

Lists are mutable, so you can change their length without rebuilding them. The most useful distinction is one item vs several items: append(value) adds one object, whereas extend(iterable) adds each value from an iterable.

Use insert(index, value) when the new item belongs before an existing position. This differs from items[index] = value, which replaces the item there.

For deletion, remove(value) searches for a matching value and deletes its first occurrence; it raises ValueError when the value is absent. pop() removes and returns the last item; pop(index) removes and returns a specific position and raises IndexError if that position is invalid. Both operations mutate the original list.

You can use these operations inside loops and conditions to maintain a queue, a checklist, or a collection of job names. Avoid chaining a mutating method into a return statement: methods such as append(), extend(), insert(), and remove() return None.

1. Add one item or a batch

append() adds a single item. extend() takes items from another iterable one by one.

services = ["api"]
services.append("worker")
services.extend(["database", "cache"])
print(services)

Output:

['api', 'worker', 'database', 'cache']

Appending the whole second list instead would create a nested list element.

2. Insert without replacing

insert(index, value) shifts the existing items to the right.

stages = ["build", "deploy"]
stages.insert(1, "test")
print(stages)

Output:

['build', 'test', 'deploy']

The old second element is still present.

3. Remove the first matching value

When a value appears twice, remove() removes only the first copy.

flags = ["ready", "retry", "ready"]
flags.remove("ready")
print(flags)

Output:

['retry', 'ready']

A value that does not occur would cause ValueError.

4. Pop an item and keep its value

With no index, pop() removes the final item; with an index, it removes that position.

workers = ["api", "queue", "mailer"]
last_worker = workers.pop()
first_worker = workers.pop(0)
print(last_worker)
print(first_worker)
print(workers)

Output:

mailer
api
['queue']

Both calls change the original list, while returning the removed value.

1. Appending a list instead of extending it: produces one nested item rather than several top-level items.
2. Using assignment when you intend insertion: items[index] = value replaces instead of shifting.
3. Passing a string to pop(): pop() expects an index, not the value to search for.
4. Expecting remove() to delete all duplicates: it removes the first matching value only.
5. Assuming mutation methods return the updated list: use the modified list variable after calling them.