Lists & Dictionaries

The two most useful collections in Python. Lists hold an ordered sequence of items. Dictionaries hold lookup pairs — a key and a value.

Lists

Square brackets, items separated by commas:

fruits = ["apple", "banana", "cherry"]
print(fruits)
print(len(fruits))   # number of items: 3

Indexing — getting an item by position

print(fruits[0])    # apple    — counting starts at 0!
print(fruits[1])    # banana
print(fruits[-1])   # cherry   — negative numbers count from the end

Slicing — getting a chunk

print(fruits[0:2])  # ['apple', 'banana']  — items 0 and 1
print(fruits[:2])   # same as above
print(fruits[1:])   # ['banana', 'cherry']

Changing a list

fruits.append("date")        # add to the end
fruits.insert(0, "apricot")   # insert at position 0
fruits.remove("banana")        # remove first matching item
fruits[1] = "avocado"            # overwrite item at position 1

print(fruits)

Useful list things

nums = [3, 1, 4, 1, 5, 9]

print(sum(nums))       # 23
print(min(nums))       # 1
print(max(nums))       # 9
print(sorted(nums))    # [1, 1, 3, 4, 5, 9]
print(5 in nums)      # True   — membership check

Looping over a list

for fruit in fruits:
    print("-", fruit)

Dictionaries

Curly braces, with key: value pairs. Use a dict when each item has a meaningful label.

person = {
    "name": "Sam",
    "age": 34,
    "email": "sam@example.com",
}

print(person["name"])     # Sam
print(person["age"])      # 34

Adding, updating, removing

person["city"] = "Belfast"     # add new key
person["age"] = 35            # update existing key
del person["email"]           # remove a key

print(person)

Safe lookups with .get()

If you ask for a key that doesn't exist with square brackets, you get a KeyError. .get() returns None instead (or whatever default you supply):

print(person.get("phone"))           # None
print(person.get("phone", "unknown"))  # unknown

Looping over a dictionary

for key in person:
    print(key, "=>", person[key])

# Or get key + value at once:
for key, value in person.items():
    print(f"{key}: {value}")

When to use which?

• List — when order matters and items are similar (a list of names, prices, scores).
• Dictionary — when each item has a label and you want to look it up by that label (a person record, a config, a count of words).

A worked example: counting words

text = "the cat sat on the mat the mat was warm"
counts = {}

for word in text.split():
    counts[word] = counts.get(word, 0) + 1

print(counts)