Lists and dicts, and knowing which one you need

A list is things in a row

prices = [120, 340, 90]
print(prices[0])      # 120
print(prices[2])      # 90
print(len(prices))    # 3

prices.append(75)
print(prices)         # [120, 340, 90, 75]
print(sum(prices))    # 625

Square brackets, commas between items. Positions start at zero, so the third item is prices[2]. This feels wrong for about a week and then feels normal.

Ask for a position that does not exist and Python stops:

print(prices[9])

IndexError: list index out of range

A list keeps its order, allows duplicates, and can be changed after it is made. prices[0] = 130 replaces the first item.

A dict is things with labels

student = {"name": "Amara", "city": "Kano", "marks": 78}
print(student["city"])     # Kano

student["marks"] = 81      # change one
student["year"] = 2        # add a new one
print(student)

Curly braces, and each entry is a key and a value separated by a colon. You look things up by key, not by position. student[0] does not work here, because there is no position zero; there is a slot called "name".

Ask for a key that is not there and Python stops:

print(student["marks_2"])

KeyError: 'marks_2'

Safer, when you are not sure:

print(student.get("marks_2"))          # None
print(student.get("marks_2", 0))       # 0
print("city" in student)               # True

A key names one slot

This matters more than it looks:

votes = {"yes": 3, "no": 1, "yes": 5}
print(votes)        # {'yes': 5, 'no': 1}
print(len(votes))   # 2

Writing "yes" twice did not store two entries and did not raise an error. The second write landed in the same slot and replaced what was there. A dict is a set of named boxes, and a name refers to exactly one box.

Choosing between them

Use a list when the items are the same kind of thing and their order or count matters. Four prices. Twelve months of rainfall. Every message in a conversation.

Use a dict when each piece has a different job and you will fetch it by name. One student's name, city and marks. One product's title, price and stock.

The honest answer to "which one" is usually: both, nested. This is the shape you will meet everywhere in AI work:

messages = [
    {"role": "user", "content": "Explain gravity in one sentence."},
    {"role": "assistant", "content": "Things with mass pull on each other."},
    {"role": "user", "content": "Now in Hindi."},
]

print(len(messages))                 # 3
print(messages[0]["content"])        # Explain gravity in one sentence.
print(messages[-1]["role"])          # user

A list, because the order of a conversation is the whole point and there can be any number of turns. Dicts inside it, because a turn has two named parts. Read messages[0]["content"] left to right: take the list, take item zero, that is a dict, take its "content" slot.

messages[-1] is the last item. Negative positions count from the end, which saves you writing messages[len(messages) - 1].

Adding a turn

messages.append({"role": "assistant", "content": "गुरुत्वाकर्षण..."})
print(len(messages))   # 4

When you call an AI API in lesson ten, you will send almost exactly this structure. You already know how to build it.

Try this now

basket = [
    {"item": "rice", "price": 850, "qty": 2},
    {"item": "oil", "price": 1200, "qty": 1},
]
print(basket[1]["item"])
basket[0]["qty"] = 3
print(basket[0])