Add & Remove: append, insert, pop, remove, del

By the end of this lesson you can:

• Grow a list with append() and insert().
• Shrink it with pop(), remove() and del, choosing the right one.
• Explain why removing items while looping over a list skips some of them.

Lesson 2 said index assignment can only replace. Here is what that means when you try to add.

queue = ["Mia", "Leo"]

queue[2] = "Ana"
print(queue)

Traceback (most recent call last):
IndexError: list assignment index out of range

A queue at the canteen. People join the back, sometimes squeeze in partway, get served from the front, or leave because they were called away. Each is a different operation.

Adding

queue = ["Mia", "Leo"]

queue.append("Ana")
print(queue)

queue.insert(0, "Teacher Rahim")
print(queue)

queue.extend(["Hugo", "Ella"])
print(queue)

Output

['Mia', 'Leo', 'Ana']
['Teacher Rahim', 'Mia', 'Leo', 'Ana']
['Teacher Rahim', 'Mia', 'Leo', 'Ana', 'Hugo', 'Ella']

• append(x) — one item on the end. The one you will use most.
• insert(i, x) — one item at position i, pushing the rest along.
• extend(other) — every item of another list, one by one.

Removing

queue = ["Teacher Rahim", "Mia", "Leo", "Ana", "Hugo"]

served = queue.pop(0)
print("Served:", served)

last = queue.pop()
print("Left early:", last)

queue.remove("Leo")
print("After remove:", queue)

del queue[0]
print("After del:", queue)

Output

Served: Teacher Rahim
Left early: Hugo
After remove: ['Mia', 'Ana']
After del: ['Ana']

Three questions decide which one you want:

• Do I need the item back? pop() returns it; remove() and del do not.
• Do I know the position or the value? pop(i) and del take a position; remove(x) takes the value.
• Am I removing a run? del items[1:3] deletes a whole slice at once.

The loop trap

This is the bug the lesson exists to prevent.

numbers = [1, 2, 2, 3, 2, 4]

for n in numbers:
    if n == 2:
        numbers.remove(n)

print(numbers)

Output

[1, 3, 2, 4]

One 2 survived. The loop walks by position while the list shrinks underneath it: remove item 1 and everything shifts left, so the next item slides into a position the loop has already passed. Never remove from a list you are looping over. Build a new list instead, or loop over a copy with for n in numbers[:].

Why it matters. Every to-do list, queue, inventory and score table grows and shrinks. These five calls are how, and the loop trap is a bug that gives no error at all.

A morning at the stall, with every tool used where it fits.

# canteen_queue.py - a queue that grows and shrinks through a break

queue = ["Mia", "Leo"]
served = []

print("Opening queue:", queue)

# Two more arrive
queue.append("Ana")
queue.append("Hugo")

# A teacher goes to the front
queue.insert(0, "Teacher Rahim")
print("After arrivals:", queue)

# Serve the first two - pop gives the name back so it can be recorded
for turn in range(2):
    person = queue.pop(0)
    served.append(person)
    print("Serving:", person)

# Leo is called to the office
if "Leo" in queue:
    queue.remove("Leo")
    print("Leo left the queue")

# A whole class joins at once
queue.extend(["Ella", "Theo", "Zoe"])

print()
print("Still waiting:", queue)
print("Already served:", served)
print(f"{len(served)} served, {len(queue)} waiting")

Output

Opening queue: ['Mia', 'Leo']
After arrivals: ['Teacher Rahim', 'Mia', 'Leo', 'Ana', 'Hugo']
Serving: Teacher Rahim
Serving: Mia
Leo left the queue
Still waiting: ['Ana', 'Hugo', 'Ella', 'Theo', 'Zoe']
Already served: ['Teacher Rahim', 'Mia']
2 served, 5 waiting

Each choice has a reason:

• pop(0) for serving — a queue is served from the front, and the name is needed for the served list.
• remove() for Leo — we know who left, not where they stood.
• in before remove — no ValueError if they had already been served.
• extend for the class — several people, added individually.

Nothing here removes from a list it is looping over. The for turn in range(2) loop counts turns, not items, which sidesteps the trap entirely.

append adds one to the end, insert adds at a position, extend adds all of another list. pop removes by position and hands the item back, remove removes the first matching value, del removes by position or slice. Never remove from a list while looping over it.