Packing List

A list is many values under one name

Every variable so far has held exactly one thing. One name, one score, one answer. A packing list is not like that. A packing list is one thing made of many things, kept in the order you wrote them down.

Python writes that with square brackets and a comma between each value:

items = ["Passport", "Charger"]

That is a single variable called items holding two pieces of text. Passport is first, Charger is second, and they will stay that way for as long as the program runs. Adding a third is a matter of typing a comma and another value inside the same brackets. The brackets are what make it a list; the commas are what keep the values apart.

A loop that hands you values, not numbers

This is the part worth slowing down on, because it is genuinely different from the loops so far.

for n in range(3) counts. It hands you 0, then 1, then 2, and every single one of them is a number. Up to now that was what a loop was.

for item in items does not count at all. It walks along the list and hands you the value sitting there — the text "Passport", then the text "Charger". Nothing is numbered, and you never say how many times to go round, because the list already knows how long it is.

for n in range(3):
    print(n)

for word in ["yes", "no", "maybe"]:
    print(word)

0
1
2
yes
no
maybe

Two loops that look almost the same and behave nothing alike. The name between for and in is yours to choose either way — n, item, word, whatever reads best — and on every pass Python puts the next thing into it for you.

len() says how many

len(items) gives back the number of values on the list, and len is short for length. It is worked out fresh each time, so a list that grows reports a bigger number on its own.

A worked example

A shed rather than a suitcase, so the answer to this one stays yours to write:

colours = ["red", "green", "blue"]

for colour in colours:
    print(f"Paint the shed {colour}")

print(f"{len(colours)} tins on the shelf")

Paint the shed red
Paint the shed green
Paint the shed blue
3 tins on the shelf

Three values on the list, three lines of output, then one line that counted them. Put a fourth colour in the brackets and you get a fourth line and a 4 at the bottom, with nothing else edited.

Your turn

The editor holds a checklist with two items on it. Press Run before changing anything.

The loop is already doing its job: two items on the list, two lines of output. But both lines read [ ] item, because the line inside the loop prints the word item in plain quotes rather than the value the variable is holding. Quotes mean plain text, and plain text is exactly what came out.

Two things to do:

1. Rewrite the line inside the loop so it prints the value instead, using an f-string like the one in the worked example. Keep the [ ] box and the space after it. Those square brackets are only a tick-box drawn in text — they sit inside quotes, so they make no list.
2. Add "Socks" and "Towel" to the list, in that order, inside the square brackets.

The footer needs no attention at all. It is already counting the list, so it will say 4 by itself the moment the two new items are in place. That is what len is for.

If something goes wrong

The likeliest slip is a missing f in front of the quote. You will spot it straight away, because the output reads [ ] {item} word for word, braces and all, on every line. Nothing has broken; the quote simply needs an f in front of it.

If one line reads [ ] PassportCharger and the footer is one short, a comma went missing between two items inside the brackets. Python does not call that an error — it joins the two names into one value — so the checklist itself is your only warning.

And if the program stops with TypeError: 'list' object cannot be interpreted as an integer, then range has found its way into the for line. range wants a number to count up to, and a list is not a number. Take it out; for item in items needs nothing else.