Lesson 4 reading

The lesson gave you a list and four things to do with it: loop over it, append, remove, and index into it. But a list is more alive than that. This reading is a tour of the rest — all of it about the same two questions: how do I ask a list something, and how do I change it?

in — asking a yes/no question

You can ask a list whether it contains something, and it answers with a boolean — the same True/False from Lesson 2:

names = ["scott", "rish", "tom", "jerry"]
print("tom" in names)
print("zoe" in names)

True
False

That's not just a party trick — it fixes a real crash. In the lesson, remove deleted a name by value. But watch what happens when the name isn't there:

names = ["scott", "rish", "tom", "jerry"]
names.remove("zoe")

Traceback (most recent call last):
  File "team.py", line 2, in <module>
    names.remove("zoe")
ValueError: list.remove(x): x not in list

Same lesson as the IndexError: the error is a hint. You asked Python to remove something that wasn't there. And you already know the fix — guard it with an if, exactly like the index guard from the lesson:

player_to_remove = input("player to remove: ")

if player_to_remove in names:
    names.remove(player_to_remove)
else:
    print("no such player")

print(names)

Now a typo prints a friendly message instead of crashing. in before remove is the safe habit.

Negative index — reaching from the end

The lesson ended on a sore spot: the last item sits at index len - 1, not len, and forgetting that earns you an IndexError. Here's the relief — Python lets you count backwards with a minus sign:

names = ["scott", "rish", "tom", "jerry", "john"]
print(names[-1])
print(names[-2])

john
jerry

names[-1] is always the last item, no matter how long the list is — no len, no subtraction, no off-by-one. names[-2] is the second-to-last. Read the minus as "from the end": -1 is the end, -2 one step back from it.

Changing an item in place

So far a list could grow (append) and shrink (remove). Here's the third power: you can edit an item that's already there. It looks just like assigning a variable — because it is:

names = ["scott", "rish", "tom", "jerry"]
names[0] = "scotty"
print(names)

['scotty', 'rish', 'tom', 'jerry']

names[0] = "scotty" reaches into slot 0 and stores a new value there, right over the old one. This is a genuinely new idea, and it's worth naming: a list can be changed. A string cannot — try to edit one letter and Python refuses:

name = "scott"
name[0] = "S"

TypeError: 'str' object does not support item assignment

That difference has a name. Lists are mutable (changeable); strings are immutable (fixed). You don't need the vocabulary yet — just remember that names[0] = ... works and name[0] = ... does not.

pop and insert — the by-position twins

append and remove work by value — add this name, delete that name. Their twins work by position, using an index.

insert drops an item in at a chosen spot, sliding everything after it down:

names = ["scott", "rish", "tom", "jerry"]
names.insert(1, "mo")
print(names)

['scott', 'mo', 'rish', 'tom', 'jerry']

pop removes the item at an index — and hands it back to you:

names = ["scott", "rish", "tom", "jerry"]
removed = names.pop(0)
print(removed)
print(names)

scott
['rish', 'tom', 'jerry']

So the toolkit has two pairs: append/remove think in names, insert/pop think in positions. Reach for whichever matches how you're thinking about the change.

Try it

Before running each one, predict the exact output — every name, every bracket. Then run it and check yourself:

names = ["scott", "rish", "tom", "jerry"]
print("rish" in names)
print(names[-1])

names = ["scott", "rish", "tom", "jerry"]
names[2] = "tommy"
names.append("john")
print(names)

names = ["scott", "rish", "tom"]
first = names.pop(0)
names.insert(0, "captain")
print(first)
print(names)