Defining and Calling Functions

A function is a reusable block of code that performs a specific task. You define a function using the def keyword, and then call it by using its name followed by parentheses.

# Defining a function
def greet():
    print("Hello! Welcome to Python.")
    print("Let's learn together!")

# Calling the function
greet()        # Runs the code inside the function
greet()        # Can be called multiple times

# Function with parameters
def greet_person(name):
    print(f"Hello, {name}! Welcome to Python.")

greet_person("Alice")
greet_person("Bob")

Output: Hello, Alice! Welcome to Python. | Hello, Bob! Welcome to Python.

Return Values and Multiple Parameters

Functions can return a value back to the code that called them using the return keyword. This lets you use the result in calculations, assignments, or other functions.

# Function that returns a value
def calculate_area(length, width):
    area = length * width
    return area

# Using the returned value
room_area = calculate_area(5, 3)
print(f"Room area: {room_area} m2")  # Room area: 15 m2

# Function with multiple return values
def get_min_max(numbers):
    return min(numbers), max(numbers)

lowest, highest = get_min_max([4, 8, 1, 9, 3])
print(f"Min: {lowest}, Max: {highest}")

Scope and Default Arguments

Scope determines where a variable can be accessed. Variables created inside a function are local and only exist within that function. Default arguments provide fallback values when a parameter is not given.

# Default arguments
def calculate_tip(bill, tip_percent=15):
    tip = bill * tip_percent / 100
    return round(tip, 2)

print(calculate_tip(50))       # Uses default 15%: $7.5
print(calculate_tip(50, 20))   # Uses 20%: $10.0

# Scope example
message = "I am global"   # Global variable

def show_scope():
    message = "I am local"  # Local variable
    print(message)          # Prints: I am local

show_scope()
print(message)              # Prints: I am global

Key point: The local variable inside the function does not affect the global variable outside.

Key Vocabulary

Function

A reusable block of code defined with def that performs a specific task.

Parameter

A variable listed in the function definition that receives a value when the function is called.

Return Value

The value a function sends back using the return keyword.

Scope

The region of code where a variable is accessible. Local scope is inside a function; global is outside.

Worked Examples

1

Create a function to calculate a student's grade

def get_grade(score):
    if score >= 90:
        return "A"
    elif score >= 80:
        return "B"
    elif score >= 70:
        return "C"
    elif score >= 50:
        return "D"
    else:
        return "F"

# Test the function
students = {"Alice": 92, "Bob": 75, "Charlie": 48}
for name, score in students.items():
    grade = get_grade(score)
    print(f"{name}: {score}% = Grade {grade}")

Step 1: The function takes a score and returns a grade letter.

Step 2: We call it in a loop for each student. Output: Alice: 92% = Grade A | Bob: 75% = Grade C | Charlie: 48% = Grade F

2

Function with default argument for temperature conversion

def convert_temp(value, to_unit="celsius"):
    if to_unit == "celsius":
        result = (value - 32) * 5 / 9
        return round(result, 1)
    elif to_unit == "fahrenheit":
        result = value * 9 / 5 + 32
        return round(result, 1)

print(convert_temp(100))                # 100F to C: 37.8
print(convert_temp(37.8, "fahrenheit")) # 37.8C to F: 100.0

Step 1: The default parameter to_unit="celsius" means if you omit it, conversion goes to Celsius.

Step 2: Passing "fahrenheit" overrides the default.

3

Function that returns statistics about a list

def get_stats(numbers):
    total = sum(numbers)
    count = len(numbers)
    average = total / count
    return {
        "count": count,
        "total": total,
        "average": round(average, 2),
        "min": min(numbers),
        "max": max(numbers)
    }

scores = [85, 92, 78, 95, 88]
stats = get_stats(scores)
for key, value in stats.items():
    print(f"{key}: {value}")

Step 1: The function calculates multiple statistics and returns a dictionary.

Step 2: We can access each stat from the returned dictionary.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Which keyword is used to define a function in Python?

function func define def

Question 2

What does a function return if there is no return statement?

None 0 False Error

Question 3

Given def add(a, b=10): return a + b, what does add(5) return?

5 15 10 Error

Question 4

What is a variable created inside a function called?

Global variable Private variable Local variable Static variable

Question 5

What is the difference between a parameter and an argument?

They are the same thing Parameters are only used in while loops An argument is in the definition; a parameter is the value passed when calling A parameter is in the definition; an argument is the value passed when calling

Python Functions