0016-dictionary-in-python.md

Dictionary in Python

A dictionary in Python is a collection of key-value pairs, where each key maps to a specific value.
Dictionaries are:

• Unordered (no fixed position)
• Mutable (can be changed)
• Indexed by keys (not by position)
• Fast for lookups (use hashing internally)

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

Keys must be unique and immutable (like strings, numbers, or tuples).
Values can be of any data type — even lists or other dictionaries.

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1. Creating Dictionaries

Empty Dictionary

empty_dict = {}
print(empty_dict)

Output:

{}

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Dictionary with Key-Value Pairs

person = {
    "name": "John",
    "age": 30,
    "city": "New York"
}
print(person)

Output:

{'name': 'John', 'age': 30, 'city': 'New York'}

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2. Accessing Dictionary Values

Access values using keys or the safer get() method.

print(person["name"])     # Direct access
print(person.get("age"))  # Using get()
print(person.get("gender", "Not specified"))  # Default value if key missing

Output:

John
30
Not specified

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3. Modifying Dictionaries

Adding a New Key-Value Pair

person["gender"] = "Male"
print(person)

Output:

{'name': 'John', 'age': 30, 'city': 'New York', 'gender': 'Male'}

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Updating an Existing Value

person["age"] = 31
print(person)

Output:

{'name': 'John', 'age': 31, 'city': 'New York', 'gender': 'Male'}

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Removing a Key-Value Pair

del person["city"]
print(person)

Output:

{'name': 'John', 'age': 31, 'gender': 'Male'}

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4. Common Dictionary Methods

Method	Description	Example	Output
keys()	Returns all keys	person.keys()	dict_keys(['name', 'age', 'gender'])
values()	Returns all values	person.values()	dict_values(['John', 31, 'Male'])
items()	Returns key-value pairs as tuples	person.items()	dict_items([('name', 'John'), ('age', 31), ('gender', 'Male')])
pop(key)	Removes the key and returns its value	person.pop('age')	31
update()	Updates dictionary with another dictionary	person.update({'city': 'SF'})	Merged result

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Example:

person.update({"age": 32, "city": "San Francisco"})
print(person)

Output:

{'name': 'John', 'gender': 'Male', 'age': 32, 'city': 'San Francisco'}

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5. Different Types of Keys and Values

Dictionaries allow keys and values of different types.

my_dict = {
    1: "one",
    "two": 2,
    (3, 4): [1, 2, 3],
    "nested_dict": {"a": 1, "b": 2}
}

Accessing Values:

print(my_dict[1])
print(my_dict["two"])
print(my_dict[(3, 4)])
print(my_dict["nested_dict"]["a"])

Output:

one
2
[1, 2, 3]
1

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6. Lists or Tuples as Values

student_scores = {
    "Alice": [85, 90, 92],
    "Bob": (88, 79, 95),
    "Charlie": [78, 81, 85]
}

print(student_scores["Alice"])
print(student_scores["Bob"][1])

Output:

[85, 90, 92]
79

---

7. Tuples as Keys

Since tuples are immutable, they can be dictionary keys.

coordinates = {
    (0, 0): "Origin",
    (1, 2): "Point A",
    (3, 4): "Point B"
}
print(coordinates[(1, 2)])

Output:

Point A

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8. Looping Through a Dictionary

for key in person:
    print(key)

for value in person.values():
    print(value)

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

Output (order may vary):

name
gender
age
city

John
Male
32
San Francisco

name: John
gender: Male
age: 32
city: San Francisco

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9. Dictionary Comprehension

Compact way to create dictionaries.

squares = {x: x**2 for x in range(1, 6)}
print(squares)

Output:

{1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

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10. Nested Dictionaries

Dictionaries can contain other dictionaries.

students = {
    "Alice": {"age": 24, "grade": "A"},
    "Bob": {"age": 22, "grade": "B"},
    "Charlie": {"age": 23, "grade": "C"}
}

print(students["Alice"]["age"])
print(students["Charlie"]["grade"])

Output:

24
C

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11. Handling Missing Keys — setdefault()

Adds the key with a default value if it doesn’t exist.

person.setdefault("nationality", "Unknown")
print(person)

Output:

{'name': 'John', 'gender': 'Male', 'age': 32, 'city': 'San Francisco', 'nationality': 'Unknown'}

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Common Errors

Error	Cause	Example
KeyError	Accessing a non-existent key directly	person["salary"]
TypeError	Using a mutable object as a key	{[1,2]: "list"}
AttributeError	Using non-existent methods	person.append("age")

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Summary

Concept	Description
Key-Value Pair	Maps unique keys to values
Mutable	Can add, remove, or modify items
Unordered	Keys are not stored in order (before Python 3.7)
Hashable Keys	Keys must be immutable (string, number, tuple)
Supports Nesting	Dictionaries can hold other dictionaries
Useful Methods	keys(), values(), items(), pop(), update(), setdefault()

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Practice Tasks

1. Create a dictionary to store student names and their grades, then print all students with grade “A”.
2. Merge two dictionaries using .update() and print the result.
3. Use a dictionary comprehension to map numbers (1–5) to their cubes.
4. Create a nested dictionary for three employees containing their name, department, and salary.
5. Try accessing a missing key using both [] and .get() — note the difference.