Dictionaries

collection = single "variable" used to store multiple values
Dictionary = {key: value} → unordered, changeable, no duplicate keys
Keys must be unique and immutable (e.g., strings, numbers)
Dictionary is great for:

• Storing key-value pairs like user data, config settings
• Fast lookup by key
• Organizing structured info

Creating a dictionary

dictionaries.py

person = {
    "name": "Hari",
    "age": 22,
    "location": "India"
}

Accessing values

dictionaries.py

person = {
    "name": "Hari",
    "age": 22,
    "location": "India"
}

print(person["name"])        # → "Hari"
print(person.get("age"))     # → 22
print(person.get("email"))   # → None (safe way to access non-existing key)

Adding or updating values

dictionaries.py

person = {
    "name": "Hari",
    "age": 22,
    "location": "India"
}

person["email"] = "hari@example.com"     # Adds new key-value
person["age"] = 23                       # Updates existing key

Removing items

dictionaries.py

person = {
    "name": "Hari",
    "age": 22,
    "location": "India"
}

del person["location"]                  # Deletes "location"
removed = person.pop("email")           # Removes and returns "email"
print("Removed:", removed)

# popitem() removes the last inserted pair (Python 3.7+)
last_item = person.popitem()
print("Last item removed:", last_item)

Clearing the dictionary

dictionaries.py

person = {
    "name": "Hari",
    "age": 22,
    "location": "India"
}

person.clear()      # → {}

Dictionary methods

dictionaries.py

person = {
    "name": "Hari",
    "age": 22,
    "location": "India"
}

print(person.keys())       # → dict_keys(['name', 'age', 'location'])
print(person.values())     # → dict_values(['Hari', 22, 'India'])
print(person.items())      # → dict_items([('name', 'Hari'), ('age', 22), ('location', 'India')])

Looping through dictionary

dictionaries.py

person = {
    "name": "Hari",
    "age": 22,
    "location": "India"
}

for key in person:
    print(key, "→", person[key])

Copying a dictionary

dictionaries.py

person = {
    "name": "Hari",
    "age": 22,
    "location": "India"
}

person_copy = person.copy()

Nested dictionary

dictionaries.py

student = {
    "name": "Hari",
    "grades": {
        "math": 85,
        "science": 90
    }
}
print(student["grades"]["math"])  # → 85

Modern Dictionary Merging

Prior to Python 3.9, merging dictionaries required using .update() or unpacking ({**d1, **d2}). Python 3.9 introduced:

• Merge operator (|): returns a new dictionary.
• Update operator (|=): merges in-place.

Dictionary Merging and Defaults

dict_a = {"x": 1, "y": 2}
dict_b = {"y": 99, "z": 4}  # Overlapping key 'y'

# Merge returns a new dict (values from right dict override left)
merged = dict_a | dict_b
print(merged)  # Output: {'x': 1, 'y': 99, 'z': 4}

# Using setdefault
# If key exists, returns value. If not, inserts key with default and returns default.
counts = {}
counts.setdefault("apple", 0)
counts["apple"] += 1
print(counts)  # Output: {'apple': 1}

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