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Week 4: Functions, Variable Scope, and Dictionaries

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Week 4: Functions, Variable Scope, and Dictionaries

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Course: Jan 2026 - Python Difficulty: Advanced Setup Focus: Code reusability, function parameters, mapping key-values.

1. Functions

A function is a reusable block of code designed to perform a specific task. Functions make code modular, easier to read, and eliminate repetition.

1.1 Defining and Calling

Use the def keyword to create a function.


python
# Definition
def greet(name):
    print(f"Hello, {name}!")

# Calling functions
greet("Alice")
greet("Bob")

1.2 Arguments and Return Values

Functions can take inputs (parameters/arguments) and send data back (return values).


python
def add_numbers(a, b):
    result = a + b
    return result

sum_val = add_numbers(5, 7)
print(sum_val) # Output: 12

Note: A function terminates immediately upon hitting a return statement.

1.3 Default Arguments

You can provide default values for parameters, making them optional during the call.


python
def power(base, exponent=2):
    return base ** exponent

print(power(3))    # Output: 9 (defaults to squaring)
print(power(3, 3)) # Output: 27

2. Variable Scope

Scope defines where in your code a variable can be accessed.

Local Scope: Variables created inside a function. They only exist while the function is running.
Global Scope: Variables created outside all functions. They are accessible anywhere in the file.


python
x = 10 # Global

def my_func():
    y = 5   # Local
    print(x) # Accessing global. Prints 10

my_func()
# print(y) <-- ERROR: 'y' is not defined in the global scope

To modify a global variable inside a function, you must use the global keyword.

3. Dictionaries

A Dictionary is an unordered, mutable collection of key-value pairs. It allows for fast retrieval of data based on a unique key. They are defined using curly braces {}.

3.1 Creating and Accessing

Keys must be immutable types (strings, numbers, tuples). Values can be anything.


python
student = {
    "name": "John",
    "age": 20,
    "courses": ["Math", "Physics"]
}

print(student["name"]) # Output: 'John'

3.2 Modifying and Adding Elements


python
# Modifying
student["age"] = 21

# Adding a new key-value pair
student["grade"] = "A"

3.3 Dictionary Methods

keys(): Returns a list-like object of all keys.
values(): Returns all values.
items(): Returns tuples of (key, value).
get(key, default): Safely gets a value. If the key doesn't exist, it returns the default value instead of crashing.

3.4 Iterating Over Dictionaries


python
# Iterating over keys
for key in student:
    print(key, "->", student[key])

# Iterating over items
for k, v in student.items():
    print(f"{k}: {v}")

Use

.get() when you aren't absolutely sure a key exists. student["address"] crashes if address is missing, whereas student.get("address", "Unknown") gracefully returns "Unknown".

📘 Textbook Notes: Week 4 Notes
📝 Assignment: Week 4 Assignment
⬅️ Previous Week: Week 3 Notes