Python Functions Explained Simply

Why Functions?

Without functions, your code repeats itself. Imagine calculating BMI for 100 students — you'd write the same formula 100 times. Functions let you write it once and use it forever.

Your First Function

def greet():
    print("Hello!")

greet()   # Call the function
greet()   # Call again
greet()   # And again

The keyword def defines a function. The parentheses () hold parameters (none here). The indented block is the function body.

Parameters: Giving Functions Information

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

greet("Chan")    # Hello, Chan!
greet("Lee")     # Hello, Lee!

name is a parameter — a variable that holds the value passed in when you call the function.

Multiple Parameters

def introduce(name, age, school):
    print(f"My name is {name}.")
    print(f"I am {age} years old.")
    print(f"I study at {school}.")

introduce("Chan", 17, "Victoria SS")

Return Values: Getting Information Back

def double(n):
    return n * 2

result = double(5)
print(result)        # 10
print(double(100))   # 200

return sends a value back to wherever the function was called.

The Difference: print vs return

def with_print(n):
    print(n * 2)       # Shows on screen

def with_return(n):
    return n * 2       # Sends value back

x = with_print(5)       # Shows 10, x is None
y = with_return(5)      # Nothing shown, y is 10

print(x + 1)   # Error! can't add 1 to None
print(y + 1)   # 11

Realistic Example: Grade Calculator

def calculate_grade(score):
    if score >= 80:
        return "A"
    elif score >= 65:
        return "B"
    elif score >= 50:
        return "C"
    elif score >= 35:
        return "D"
    else:
        return "F"

# Use the function
students = [("Chan", 85), ("Lee", 72), ("Wong", 45)]
for name, score in students:
    grade = calculate_grade(score)
    print(f"{name}: {grade}")

Default Parameters

def mtr_fare(distance, is_student=True):
    fare = distance * 0.5
    if is_student:
        fare *= 0.5   # 50% discount
    return fare

print(mtr_fare(10))              # 2.5 (student)
print(mtr_fare(10, False))       # 5.0 (adult)

Multiple Return Values

def get_stats(numbers):
    return min(numbers), max(numbers), sum(numbers)/len(numbers)

lowest, highest, average = get_stats([85, 72, 91, 68])
print(f"Lowest: {lowest}, Highest: {highest}, Average: {average:.1f}")

Best Practices

• Use lowercase_with_underscores for function names
• Name should describe what it does (calculate_grade, not cg)
• Each function should do one thing well
• Use return when the caller needs the result
• Add a short comment if the purpose isn't obvious