Lists and Dictionaries in Python: Working with Data 📦

So far: one variable = one value.

Problem: Want to store 1000 students → 1000 variables? 😱

Solution: Collections — lists and dictionaries for storing multiple pieces of data!

📋 Lists — ordered collections

A list is a sequence of elements stored under one name.

Creating lists:

# Empty list:
empty_list = []

# List of numbers:
numbers = [1, 2, 3, 4, 5]

# List of strings:
fruits = ["apple", "banana", "orange"]

# Mixed list (different types):
mixed = ["text", 42, True, 3.14, [1, 2, 3]]

Accessing elements (indexing):

⚠️ IMPORTANT: Indexes start at 0!

fruits = ["apple", "banana", "orange"]
#          0          1          2

print(fruits[0])   # apple (first!)
print(fruits[1])   # banana
print(fruits[2])   # orange
print(fruits[-1])  # orange (last)
print(fruits[-2])  # banana (second to last)

Modifying elements:

fruits[1] = "pear"  # Replace the second element
print(fruits)  # ["apple", "pear", "orange"]

List length:

print(len(fruits))  # 3 elements

---

➕ Adding and removing elements

append() — add to the end:

fruits = ["apple", "banana"]
fruits.append("orange")
print(fruits)  # ["apple", "banana", "orange"]

fruits.append("pear")
print(fruits)  # ["apple", "banana", "orange", "pear"]

insert() — insert at a position:

fruits = ["apple", "orange"]
fruits.insert(1, "banana")  # Insert at index 1
print(fruits)  # ["apple", "banana", "orange"]

remove() — remove by value:

fruits = ["apple", "banana", "orange"]
fruits.remove("banana")
print(fruits)  # ["apple", "orange"]

Removes the first occurrence!

pop() — remove by index (and return the value):

fruits = ["apple", "banana", "orange"]

# Remove the last element:
last = fruits.pop()
print(last)    # orange
print(fruits)  # ["apple", "banana"]

# Remove by index:
first = fruits.pop(0)
print(first)   # apple
print(fruits)  # ["banana"]

clear() — clear the entire list:

fruits.clear()
print(fruits)  # []

---

🔪 Slicing — subsets of a list

Syntax: list[start:end:step]

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

# Elements from index 2 up to (not including) 5:
print(numbers[2:5])  # [2, 3, 4]

# From the beginning up to index 5:
print(numbers[:5])   # [0, 1, 2, 3, 4]

# From index 5 to the end:
print(numbers[5:])   # [5, 6, 7, 8, 9]

# All elements (a copy):
print(numbers[:])    # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

# Every second element:
print(numbers[::2])  # [0, 2, 4, 6, 8]

# Every third element:
print(numbers[::3])  # [0, 3, 6, 9]

# Reverse the list:
print(numbers[::-1]) # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

# Last 3 elements:
print(numbers[-3:])  # [7, 8, 9]

# First 3 elements:
print(numbers[:3])   # [0, 1, 2]

Slices create a NEW list — the original is not modified!

---

🔧 Useful list methods

sort() — sort in place:

numbers = [3, 1, 4, 1, 5, 9, 2]
numbers.sort()
print(numbers)  # [1, 1, 2, 3, 4, 5, 9]

# Reverse sort:
numbers.sort(reverse=True)
print(numbers)  # [9, 5, 4, 3, 2, 1, 1]

reverse() — reverse in place:

letters = ['A', 'B', 'C', 'D']
letters.reverse()
print(letters)  # ['D', 'C', 'B', 'A']

count() — count occurrences:

numbers = [1, 2, 3, 2, 4, 2, 5]
print(numbers.count(2))  # 3 (the value 2 appears three times)

index() — find the index of an element:

fruits = ["apple", "banana", "orange"]
index = fruits.index("banana")
print(index)  # 1

extend() — add multiple elements:

list1 = [1, 2, 3]
list2 = [4, 5, 6]
list1.extend(list2)
print(list1)  # [1, 2, 3, 4, 5, 6]

Membership test (in):

fruits = ["apple", "banana", "orange"]

if "banana" in fruits:
    print("We have bananas!")  # Prints

if "pear" not in fruits:
    print("No pears")          # Prints

---

📖 Dictionaries — key-value pairs

Lists: access by index (0, 1, 2…)
Dictionaries: access by key (name, age, email…)

Creating dictionaries:

# Empty dictionary:
empty_dict = {}

# Dictionary with data:
student = {
    "name": "Anna",
    "age": 16,
    "city": "Moscow",
    "grade": 10
}

# Keys can be strings or numbers:
scores = {
    1: 100,
    2: 85,
    3: 92
}

Accessing values:

student = {
    "name": "Anna",
    "age": 16,
    "city": "Moscow"
}

print(student["name"])  # Anna
print(student["age"])   # 16
print(student["city"])  # Moscow

Modifying values:

student["age"] = 17
student["grade"] = 11
print(student)  # Age and grade updated!

Adding new keys:

student["email"] = "anna@example.com"
student["phone"] = "+1-123-456-7890"
print(student)  # email and phone added

Removing keys:

# del — remove a key:
del student["phone"]

# pop() — remove and return the value:
email = student.pop("email")
print(email)  # anna@example.com
print(student)  # email removed

---

🔑 Dictionary methods

get() — safe lookup (no KeyError):

student = {"name": "Anna", "age": 16}

# Direct access (can raise KeyError):
print(student["email"])  # ❌ KeyError!

# Safe access:
email = student.get("email", "No email")
print(email)  # "No email" (default value)

age = student.get("age", 0)
print(age)  # 16 (key exists)

keys() — all keys:

student = {"name": "Anna", "age": 16, "city": "Moscow"}

keys = student.keys()
print(keys)  # dict_keys(['name', 'age', 'city'])
print(list(keys))  # ['name', 'age', 'city']

values() — all values:

values = student.values()
print(values)  # dict_values(['Anna', 16, 'Moscow'])
print(list(values))  # ['Anna', 16, 'Moscow']

items() — key-value pairs:

items = student.items()
print(items)
# dict_items([('name', 'Anna'), ('age', 16), ('city', 'Moscow')])

# Iteration:
for key, value in student.items():
    print(f"{key}: {value}")
# name: Anna
# age: 16
# city: Moscow

update() — update multiple keys:

student = {"name": "Anna", "age": 16}

student.update({
    "age": 17,
    "city": "Moscow",
    "grade": 11
})

print(student)
# {"name": "Anna", "age": 17, "city": "Moscow", "grade": 11}

clear() — clear the dictionary:

student.clear()
print(student)  # {}

Checking key membership:

if "name" in student:
    print("Name is present!")

if "email" not in student:
    print("No email field")

---

🔄 Iterating over lists and dictionaries

Lists:

fruits = ["apple", "banana", "orange"]

# Simple iteration:
for fruit in fruits:
    print(fruit)

# With index (enumerate):
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")
# 0: apple
# 1: banana
# 2: orange

# Starting index at 1:
for i, fruit in enumerate(fruits, 1):
    print(f"{i}. {fruit}")
# 1. apple
# 2. banana
# 3. orange

Dictionaries:

student = {"name": "Anna", "age": 16, "city": "Moscow"}

# Keys only:
for key in student:
    print(key)
# name
# age
# city

# Values only:
for value in student.values():
    print(value)
# Anna
# 16
# Moscow

# Keys and values:
for key, value in student.items():
    print(f"{key}: {value}")
# name: Anna
# age: 16
# city: Moscow

---

🎯 Practical examples

1. TODO list:

tasks = []

# Adding:
tasks.append("Study")
tasks.append("Exercise")
tasks.append("Reading")

# Displaying:
print("📝 My tasks:")
for i, task in enumerate(tasks, 1):
    print(f"{i}. {task}")

# Removing:
tasks.remove("Exercise")

# Checking:
if "Study" in tasks:
    print("Study is still pending!")

2. Vote counting:

votes = ["Python", "JavaScript", "Python", "Java", "Python", "JavaScript"]

# Count:
vote_count = {}
for vote in votes:
    if vote in vote_count:
        vote_count[vote] += 1
    else:
        vote_count[vote] = 1

print(vote_count)
# {'Python': 3, 'JavaScript': 2, 'Java': 1}

# Winner:
winner = max(vote_count, key=vote_count.get)
print(f"Winner: {winner}")

3. Student database:

students = [
    {"name": "Anna", "age": 16, "grade": 10},
    {"name": "Bob", "age": 17, "grade": 11},
    {"name": "Sveta", "age": 16, "grade": 10}
]

# Display all:
for student in students:
    print(f"{student['name']}, age {student['age']}, grade {student['grade']}")

# Search:
search_name = "Anna"
for student in students:
    if student["name"] == search_name:
        print(f"Found: {student}")
        break

# Filter by grade:
grade_10 = [s for s in students if s["grade"] == 10]
print(f"Grade 10: {len(grade_10)} students")

4. Game inventory:

inventory = {
    "sword": 1,
    "shield": 1,
    "health potion": 5,
    "mana potion": 3,
    "gold": 150
}

# Display inventory:
print("🎒 Inventory:")
for item, count in inventory.items():
    print(f"  {item}: {count}")

# Using an item:
if inventory["health potion"] > 0:
    inventory["health potion"] -= 1
    print("🧪 Used a health potion!")
    print(f"Remaining: {inventory['health potion']}")

# Adding gold:
inventory["gold"] += 50
print(f"💰 Gold: {inventory['gold']}")

---

📚 Nested structures

List of dictionaries:

users = [
    {"username": "anna", "age": 16, "city": "Moscow"},
    {"username": "bob", "age": 18, "city": "SPb"},
    {"username": "charlie", "age": 17, "city": "Moscow"}
]

# Access:
print(users[0]["username"])  # anna
print(users[1]["age"])       # 18

# Find users from Moscow:
moscow_users = [u for u in users if u["city"] == "Moscow"]
print(f"From Moscow: {len(moscow_users)}")

Dictionary of lists:

courses = {
    "python": ["Anna", "Bob", "Sveta"],
    "javascript": ["Pete", "Mary"],
    "java": ["Ivan", "Kate", "Alex"]
}

# Access:
print(courses["python"])  # ['Anna', 'Bob', 'Sveta']
print(courses["python"][0])  # Anna

# Add a student:
courses["python"].append("New Student")

# Number of students per course:
for course, students in courses.items():
    print(f"{course}: {len(students)} students")

Dictionary of dictionaries:

contacts = {
    "anna": {
        "phone": "+1-123-456",
        "email": "anna@mail.com",
        "city": "Moscow"
    },
    "bob": {
        "phone": "+1-987-654",
        "email": "bob@mail.com",
        "city": "SPb"
    }
}

# Access:
print(contacts["anna"]["email"])  # anna@mail.com
print(contacts["bob"]["phone"])   # +1-987-654

# Add a new contact:
contacts["charlie"] = {
    "phone": "+1-555-123",
    "email": "charlie@mail.com",
    "city": "Kazan"
}

---

⚠️ Common mistakes

1. IndexError — out of bounds:

fruits = ["apple", "banana"]
print(fruits[2])  # ❌ IndexError!

# Fix — check first:
if len(fruits) > 2:
    print(fruits[2])

2. KeyError — key not found:

student = {"name": "Anna"}
print(student["age"])  # ❌ KeyError!

# Fix — use get():
age = student.get("age", 0)
print(age)  # 0

3. Modifying a list while iterating:

# ❌ Dangerous:
numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num % 2 == 0:
        numbers.remove(num)  # May skip elements!

# ✅ Correct — build a new list:
numbers = [1, 2, 3, 4, 5]
odd_numbers = [n for n in numbers if n % 2 != 0]
print(odd_numbers)  # [1, 3, 5]

4. Copying lists:

# ❌ Reference, not a copy:
list1 = [1, 2, 3]
list2 = list1
list2[0] = 999
print(list1)  # [999, 2, 3] — changed!

# ✅ Actual copy:
list1 = [1, 2, 3]
list2 = list1.copy()  # or list1[:]
list2[0] = 999
print(list1)  # [1, 2, 3] — unchanged!

---

🚀 Summary

Lists and dictionaries are the backbone of data handling!

You’ve learned to:

✅ Create and use lists
✅ Add, remove, and modify elements
✅ Use slicing to extract subsets
✅ Work with dictionaries (key-value pairs)
✅ Apply list and dictionary methods
✅ Iterate over collections
✅ Build nested data structures

With collections you can:
- Store any amount of data
- Organize complex structures
- Build in-memory databases
- Process large volumes of information
- Solve real-world problems

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Practice in CodeHS! Build:
- TODO lists
- Address books
- Student databases
- Game inventories
- Statistics and analytics

Collections are everywhere: files, databases, APIs, web development, data science! 💪

Next step: Explore file I/O and Python libraries! 📂