lesson 33 Lists & Tuples · beginner

Change a list

Unlike text, a list can change in place. append adds an item, items[i] = x replaces one, and pop removes the last one. A tuple can't change.

You'll be able to

Add, replace and remove items, and explain that these change the list itself
Build a result list in a loop, starting from []
Avoid items = items.append(x), and explain why a tuple raises TypeError

The idea

Text never changes, and every string method builds a new text. A list is different: it can change in place. A shopping cart is a good picture of this, because the same cart gains items, has an item swapped and loses the last one.

cart.append(x) adds x at the end.
cart[i] = x replaces the item at position i.
cart.pop() removes the last item and gives it back to you.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · A cart that changes

cart = []
cart.append("P0607")
cart.append("P0208")
cart.append("P0659")
print(cart, len(cart))
cart[1] = "P0209"
print(cart)
removed = cart.pop()
print(removed, cart)

Changing items through their positions is how you repair data in place. Weather stations sometimes miss a day, and the gap arrives as None. A common fix is to carry the previous day's reading forward. Loop over positions, so you can both read the previous item and replace the current one.

example-2.py · Filling gaps in the readings

temps = [6.7, None, 3.3, None, None, 3.9]
for i in range(1, len(temps)):
    if temps[i] is None:
        temps[i] = temps[i - 1]
print(temps)

example-3.py · A tuple can't change

film = ("Wild Archive", 2008)
print(film[1])
film[1] = 2009

When to use it: to collect results in a loop (start from [] and append), and to update or repair a list that the rest of the program shares. Mistakes to avoid:

Writing cart = cart.append(x). append changes the list and returns None, so this replaces your list with None.
Calling pop() on an empty list, which raises IndexError. Check len first.

Review

Lists change in place. append adds at the end, items[i] = x replaces, and pop() removes and returns the last item.
These methods return None, so call them on their own line and don't store the result.
Build a result by starting from [] and appending in a loop. Check len before you pop.
Next: sorting, and why a second name for a list is not a copy.

Practise this