Python Sets, Dictionaries & Comparison of List, Tuple, and Set

In addition to sequential collections (str, list, tuple), Python provides two high-performance collections based on hashing: Set (for unique items) and Dictionary (for key-value pairs).

1. Set (set) in Python

A Set is an unordered, unindexed, and mutable collection of unique elements enclosed in curly braces { }.

Key Properties of Sets:

1. No Duplicate Elements: Sets automatically remove duplicate values.
2. Unordered & Unindexed: Elements do not have a fixed position, so you cannot access items using indexes like s[0].
3. Mutable: You can add or remove items from a set using .add() and .remove().

Exam Trap — Empty Set Creation: Writing s = {} creates an empty Dictionary, NOT an empty Set! To create an empty set, you must use s = set().

# Duplicate values are automatically removed
nums = {10, 20, 20, 30, 10, 40}
print(nums)  # {10, 20, 30, 40}

# Adding and removing elements
nums.add(50)
nums.discard(20)  # Removes 20 safely (does not raise error if 20 is missing)

Mathematical Set Operations in Python

A = {1, 2, 3}
B = {3, 4, 5}

print("Union:", A | B)                 # or A.union(B) -> {1, 2, 3, 4, 5}
print("Intersection:", A & B)          # or A.intersection(B) -> {3}
print("Difference (A - B):", A - B)    # or A.difference(B) -> {1, 2}
print("Symmetric Diff:", A ^ B)        # or A.symmetric_difference(B) -> {1, 2, 4, 5}

2. Dictionary (dict) in Python

A Dictionary is a mutable collection that stores data in key : value pairs enclosed within curly braces { }.

Key Properties of Dictionaries:

1. Key-Value Mapping: Instead of numeric indexes (0, 1, 2), items are accessed via their Key.
2. Unique & Immutable Keys: Keys must be unique and immutable (strings, numbers, or tuples). If you repeat a key, the new value overwrites the old value.
3. Mutable Values: Values can be of any data type, can be duplicated, and can be modified anytime.

Creating, Accessing & Updating a Dictionary:

student = {
    "roll_no": 101,
    "name": "Ishita",
    "course": "BCA",
    "cgpa": 8.9
}

# Accessing values
print(student["name"])          # Output: Ishita
print(student.get("course"))    # Output: BCA (Safer: returns None if key doesn't exist)

# Adding or updating a key-value pair
student["semester"] = 4         # Adds new key 'semester'
student["cgpa"] = 9.2           # Updates existing key 'cgpa'

# Iterating over a dictionary
for key, value in student.items():
    print(f"{key} : {value}")

Important Built-in Dictionary Methods:

3. Comparison of List, Tuple, Set, and Dictionary

This comparison table is one of the most frequently asked questions in DAVV BCA exams: