Python Dictionary

1. Creating a Dictionary

empty_dict = {}
student = {"name": "Alice", "age": 22, "grade": "A"}

print(empty_dict)
print(student)

Dictionaries store data as key–value pairs and are optimized for fast lookups.

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2. Accessing Dictionary Values

employee = {"id": 101, "name": "John", "role": "Developer"}

print(employee["name"])        # Output: John
print(employee.get("role"))    # Output: Developer

• [] raises KeyError if missing

• get() safely returns None by default

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3. Adding and Updating Dictionary Entries

profile = {"username": "admin"}

profile["email"] = "[e-mail]"
profile["username"] = "administrator"

print(profile)

Assigning a value to a key adds or updates it.

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4. Removing Items from a Dictionary

data = {"a": 1, "b": 2, "c": 3}

removed_value = data.pop("b")
del data["a"]

print(data)         # {'c': 3}
print(removed_value)  # 2

Used to delete keys and retrieve values.

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5. Dictionary Length and Membership

settings = {"theme": "dark", "version": 1.0}

print(len(settings))        # Output: 2
print("theme" in settings)  # True

Checks existence of keys only.

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6. Iterating Through Dictionaries

user = {"name": "Alice", "age": 30, "city": "London"}

for key in user:
    print(key, ":", user[key])

for key, value in user.items():
    print(key, "=>", value)

Supports iteration over keys, values, or both.

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7. Dictionary Methods

data = {"x": 10, "y": 20}

print(data.keys())     # dict_keys(['x', 'y'])
print(data.values())   # dict_values([10, 20])
print(data.items())    # dict_items([('x', 10), ('y', 20)])

Useful for introspection and transformation.

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8. Merging Dictionaries

dict1 = {"a": 1, "b": 2}
dict2 = {"c": 3, "d": 4}

merged = dict1 | dict2
print(merged)

Introduced in Python 3.9 for clean dictionary merging.

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9. Nested Dictionaries

employee = {
    "id": 101,
    "personal": {
        "name": "John",
        "email": "[e-mail]"
    }
}

print(employee["personal"]["name"])  # Output: John

Allows structured data modeling.

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10. Dictionary Comprehension

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

Efficient way to generate dictionaries dynamically.

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