Dictionaries and Sets¶

Python provides two built-in hash-based collection types: Dictionaries for key-value pairs, and Sets for unique collections. Both provide average $O(1)$ time complexity for lookups and insertions.

1. Dictionaries (`dict`)¶

A dictionary maps unique keys to values. Dictionaries are mutable and preserve insertion order (Python 3.7+).

Creating Dictionaries¶

# Key-value syntax using curly braces
student = {
    "id": 101,
    "name": "Alice",
    "major": "Computer Science",
    "gpa": 3.8
}

# Empty dictionary
empty_dict = {}
# or empty_dict = dict()

Accessing Values Safely (`.get()`)¶

Accessing a nonexistent key directly with square brackets raises a KeyError. Use .get() to provide a default fallback:

Example: Accessing Dictionary Keys

config = {"host": "localhost", "port": 8080}

# Safe access with default
timeout = config.get("timeout", 30)
print("Timeout:", timeout)  # 30

Common Dictionary Methods¶

Method	Description	Example
`d.get(key, default)`	Returns value for key, or default if not found	`d.get('name', 'N/A')`
`d.keys()`	Returns a view of dictionary keys	`d.keys()`
`d.values()`	Returns a view of dictionary values	`d.values()`
`d.items()`	Returns a view of `(key, value)` pairs	`d.items()`
`d.update(other)`	Updates dictionary with elements from other	`d.update({'port': 9000})`
`d.pop(key[, default])`	Removes specified key and returns value	`d.pop('host')`
`d.clear()`	Removes all elements	`d.clear()`

Merging Dictionaries (Python 3.9+ Union Operator `|`)¶

defaults = {"theme": "light", "language": "en"}
user_pref = {"theme": "dark"}

# Merge using | operator
merged = defaults | user_pref
print(merged)  # {'theme': 'dark', 'language': 'en'}

2. Sets (`set`)¶

A set is an unordered collection of distinct (unique) hashable elements. Duplicate entries are automatically eliminated.

Creating Sets¶

# Using curly braces with values (not key-value pairs)
prime_set = {2, 3, 5, 7, 2, 3}
print(prime_set)  # {2, 3, 5, 7}

# NOTE: An empty set MUST be created using set(), not {} (which creates a dict)
empty_set = set()

Removing Duplicates from a Sequence¶

raw_numbers = [1, 2, 2, 3, 4, 4, 4, 5]
unique_numbers = list(set(raw_numbers))
print(unique_numbers)  # [1, 2, 3, 4, 5]

Mathematical Set Operations¶

Operation	Operator	Method	Description
Union	`A \| B`	`A.union(B)`	Elements present in either A, B, or both
Intersection	`A & B`	`A.intersection(B)`	Elements present in BOTH A and B
Difference	`A - B`	`A.difference(B)`	Elements in A that are NOT in B
Symmetric Diff	`A ^ B`	`A.symmetric_difference(B)`	Elements in either A or B, but NOT both

Example: Set Operations

set_a = {1, 2, 3, 4}
set_b = {3, 4, 5, 6}

print("Union (|):             ", set_a | set_b)
print("Intersection (&):      ", set_a & set_b)
print("Difference (A - B):    ", set_a - set_b)
print("Symmetric Difference (^):", set_a ^ set_b)

Output

Union (|): {1, 2, 3, 4, 5, 6} Intersection (&): {3, 4} Difference (A - B): {1, 2} Symmetric Difference (^): {1, 2, 5, 6}

3. Frozen Sets (`frozenset`)¶

A frozenset is an immutable version of a Python set. Because it is immutable and hashable, it can be used as a dictionary key:

fixed_permissions = frozenset(["read", "write"])
user_roles = {fixed_permissions: "Editor"}

Dictionaries and Sets¶

1. Dictionaries (dict)¶