Python Dictionaries · Organize Data with Key-Value Pairs

• 1. What is a dictionary?
• 2. Dictionary operations and methods
• 3. Looping through dictionaries
• 4. Nested dictionaries
• 5. Dictionary vs List
• 6. Common patterns (counters, defaults)
• 7. Quick reference
• 8. Quick Challenge: Dictionary Detective
• 9. Exercises
• 10. Common Errors & Solutions
• 11. Check Your Understanding
• 12. Your Progress Tracker

A dictionary stores data in key-value pairs. Each key is unique and is used to access its corresponding value. Dictionaries are created using curly braces { }.

# Dictionary examples
student = {"name": "Alice", "age": 25, "grade": "A"}
person = {
    "first_name": "John",
    "last_name": "Doe",
    "age": 30,
    "city": "New York"
}
empty = {}

# Accessing values
print(student["name"])    # "Alice"
print(student["age"])     # 25

student = {"name": "Alice", "age": 25, "grade": "A"}

# Safe access with get() (no error if key missing!)
print(student.get("name"))        # "Alice"
print(student.get("score"))       # None (no error!)
print(student.get("score", 0))    # 0 (default value)

# Adding or updating
student["city"] = "Boston"        # add new key
student["age"] = 26               # update existing key

# Removing items
del student["grade"]              # remove specific key
age = student.pop("age")          # remove and return value
last_item = student.popitem()     # remove and return last item

# Other useful methods
student.clear()                   # remove all items
keys = student.keys()             # get all keys
values = student.values()         # get all values
items = student.items()           # get all key-value pairs
len(student)                      # number of items

# Check if key exists
if "name" in student:
    print("Name exists!")

student = {"name": "Alice", "age": 25, "city": "Boston"}

# Loop through keys
for key in student:
    print(key, student[key])

# Loop through values
for value in student.values():
    print(value)

# Loop through key-value pairs (MOST USEFUL!)
for key, value in student.items():
    print(f"{key}: {value}")

# Dictionary comprehension
squares = {x: x*x for x in range(1, 6)}
# {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Dictionaries can contain other dictionaries. This is great for complex data like game characters, students, or settings.

# Nested dictionary example
students = {
    "student1": {"name": "Alice", "age": 20, "grade": "A"},
    "student2": {"name": "Bob", "age": 22, "grade": "B"},
    "student3": {"name": "Charlie", "age": 21, "grade": "A"}
}

# Accessing nested values
print(students["student1"]["name"])    # "Alice"
print(students["student2"]["grade"])   # "B"

# Looping through nested dictionary
for student_id, info in students.items():
    print(f"{student_id}: {info['name']} - {info['grade']}")

# Game character example
game = {
    "player": {"name": "Hero", "health": 100, "position": (10, 20)},
    "enemies": [{"name": "Goblin", "health": 30}, {"name": "Orc", "health": 50}],
    "score": 0,
    "active": True
}

# When to use dictionary:
student_grades = {"Alice": 85, "Bob": 92, "Charlie": 78}  # lookup by name

# When to use list:
scores = [85, 92, 78]   # simple ordered collection

# Converting between list and dictionary
keys = ["a", "b", "c"]
values = [1, 2, 3]
dictionary = dict(zip(keys, values))   # {'a': 1, 'b': 2, 'c': 3}

# Counting letters (most common use case!)
word = "hello world"
char_count = {}
for char in word:
    char_count[char] = char_count.get(char, 0) + 1
print(char_count)   # {'h':1, 'e':1, 'l':3, 'o':2, ' ':1, 'w':1, 'r':1, 'd':1}

# Using defaultdict (even easier!)
from collections import defaultdict
counts = defaultdict(int)   # default value is 0
counts["apple"] += 1        # works even if "apple" didn't exist!

# Merging dictionaries (Python 3.5+)
dict1 = {"a": 1, "b": 2}
dict2 = {"c": 3, "d": 4}
merged = {**dict1, **dict2}   # {'a': 1, 'b': 2, 'c': 3, 'd': 4}

# Using update() method
dict1.update(dict2)   # adds dict2's items to dict1

# CREATING
d = {}                            # empty dictionary
d = {"key": "value"}              # with items
d = dict(name="Alice", age=25)    # using dict() constructor

# ACCESSING
d["key"]                          # access (raises KeyError if missing)
d.get("key")                      # access (returns None if missing)
d.get("key", default)             # with default value

# ADDING/UPDATING
d["new_key"] = "new_value"        # add or update
d.update({"k1": 1, "k2": 2})     # merge another dictionary

# REMOVING
del d["key"]                      # remove key
value = d.pop("key")              # remove and return value
d.popitem()                       # remove and return last item
d.clear()                         # remove all items

# CHECKING
"key" in d                        # check if key exists
len(d)                            # number of items

# ITERATING
for key in d:                     # loop through keys
for value in d.values():          # loop through values
for key, value in d.items():      # loop through pairs (BEST!)

# COMPREHENSION
squares = {x: x*x for x in range(5)}   # {0:0, 1:1, 2:4, 3:9, 4:16}

1. How do you create an empty dictionary?
2. How do you access a value using its key?
3. What is the difference between dict[key] and dict.get(key)?
4. How do you add a new key-value pair?
5. How do you loop through both keys and values?
6. Can you use a list as a dictionary key? Why or why not?