Python Advanced: Lists & Dictionaries

Level Goal

Today you will learn:

• Nesting lists – lists within lists
• Sorting and filtering lists
• Nesting dictionaries – complex data structures
• Checking values and iterating – loops for dictionaries

By the end, you will be able to store complex collections of data and access them efficiently.

Step 1: Nesting Lists

A list can contain other lists.

This is useful for storing multidimensional data, e.g., a color palette:

colors = [["red", "darkred"], ["green", "lightgreen"], ["blue", "lightblue"]]

print(colors[0][1])

Output:

darkred

Explanation:

• colors[0] → first list: ["red", "darkred"]
• colors[0][1] → second element of this list: "darkred"

Practical tip:

Nested lists are ideal for tables or grouped values.

You can use nested loops to read all values:

for color_group in colors:
    for color in color_group:
        print(color)

Step 2: Sorting and Filtering Lists

Lists can be sorted, reversed, or filtered for specific elements.

Sort ascending:

numbers = [5, 2, 9, 1]
numbers.sort()
print(numbers)

Output:

[1, 2, 5, 9]

Reverse order:

numbers_reversed = list(reversed(numbers))
print(numbers_reversed)

Output:

[9, 5, 2, 1]

Filter using loops or list comprehensions:

Only even numbers:

even_numbers = [x for x in numbers if x % 2 == 0]
print(even_numbers)

Output:

[2]

Practical tip:

• sort() modifies the original list
• sorted(list) → creates a new sorted list
• reversed() → just reverses order, does not sort

Step 3: Nesting Dictionaries

A dictionary can contain other dictionaries to store complex information.

people = {
    "Max": {"age": 20, "city": "Berlin"},
    "Anna": {"age": 25, "city": "Hamburg"}
}

print(people["Max"]["city"])

Output:

Berlin

Explanation:

• "Max" → key in the outer dictionary
• {"age": 20, "city": "Berlin"} → value, which is itself a dictionary
• "city" → key in the inner dictionary

Iterating over nested dictionaries:

for name, data in people.items():
    print(name, "lives in", data["city"], "and is", data["age"], "years old")

Output:

Max lives in Berlin and is 20 years old
Anna lives in Hamburg and is 25 years old

Practice Exercise 1: Nested Shopping List

Create a shopping list that contains different categories: fruits, vegetables, drinks.

Example structure:

shopping_list = {
    "Fruits": ["Apple", "Banana"],
    "Vegetables": ["Carrot", "Bell Pepper"],
    "Drinks": ["Water", "Juice"]
}

Print all items, e.g., using loops.

Add a new category or a new product.

Practice Exercise 2: Pets Dictionary

Create a dictionary for 3 pets. Each pet has: Name, Type, Age.

Example:

pets = {
    "Pet1": {"Name": "Bello", "Type": "Dog", "Age": 5},
    "Pet2": {"Name": "Miezi", "Type": "Cat", "Age": 3},
    "Pet3": {"Name": "Hoppel", "Type": "Rabbit", "Age": 2}
}

for key, pet in pets.items():
    print(pet["Name"], "-", pet["Type"])
    
# Check which pet is older than 3 years
for key, pet in pets.items():
    if pet["Age"] > 3:
        print(pet["Name"], "is older than 3 years")