Dictionaries

Dictionaries store data as key-value pairs. They're perfect for when you want to look up values by a unique identifier.

Creating Dictionaries

# Empty dictionary
empty = {}

# With values
person = {
    "name": "Alice",
    "age": 30,
    "city": "New York"
}

# Using dict() constructor
person = dict(name="Alice", age=30, city="New York")

Accessing Values

person = {"name": "Alice", "age": 30, "city": "New York"}

# Using brackets
print(person["name"])  # "Alice"

# Using get() - safer, returns None if key doesn't exist
print(person.get("name"))       # "Alice"
print(person.get("country"))    # None
print(person.get("country", "USA"))  # "USA" (default value)

Tip: Use get() when the key might not exist to avoid KeyError.

Modifying Dictionaries

person = {"name": "Alice", "age": 30}

# Add or update a key
person["city"] = "New York"  # Add new key
person["age"] = 31           # Update existing key

# Remove a key
del person["city"]

# Pop - remove and return value
age = person.pop("age")
print(age)  # 31

Dictionary Methods

Method	Description
`keys()`	Returns all keys
`values()`	Returns all values
`items()`	Returns key-value pairs as tuples
`get(key, default)`	Get value with default if missing
`pop(key)`	Remove and return value
`update(dict2)`	Merge another dictionary
`clear()`	Remove all items

person = {"name": "Alice", "age": 30, "city": "NYC"}

print(list(person.keys()))    # ["name", "age", "city"]
print(list(person.values()))  # ["Alice", 30, "NYC"]
print(list(person.items()))   # [("name", "Alice"), ...]

Looping Over Dictionaries

person = {"name": "Alice", "age": 30, "city": "NYC"}

# Loop over keys
for key in person:
    print(key)

# Loop over values
for value in person.values():
    print(value)

# Loop over both
for key, value in person.items():
    print(f"{key}: {value}")

Nested Dictionaries

students = {
    "alice": {
        "grade": "A",
        "age": 20
    },
    "bob": {
        "grade": "B",
        "age": 21
    }
}

# Access nested values
print(students["alice"]["grade"])  # "A"

Dictionary Comprehensions

# Create a dictionary of squares
squares = {x: x**2 for x in range(5)}
print(squares)  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

# Filter while creating
even_squares = {x: x**2 for x in range(10) if x % 2 == 0}
print(even_squares)  # {0: 0, 2: 4, 4: 16, 6: 36, 8: 64}

Common Use Cases

Counting occurrences

text = "hello world"
char_count = {}

for char in text:
    if char in char_count:
        char_count[char] += 1
    else:
        char_count[char] = 1

print(char_count)
# {'h': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'w': 1, 'r': 1, 'd': 1}

Grouping data

students = [
    {"name": "Alice", "grade": "A"},
    {"name": "Bob", "grade": "B"},
    {"name": "Charlie", "grade": "A"}
]

by_grade = {}
for student in students:
    grade = student["grade"]
    if grade not in by_grade:
        by_grade[grade] = []
    by_grade[grade].append(student["name"])

print(by_grade)
# {"A": ["Alice", "Charlie"], "B": ["Bob"]}

When to use dictionaries: Use them when you need fast lookups by a unique key, or when data naturally has key-value relationships.