Module 3 Lesson 2: Defining and Calling Functions

Creating a function in Python consists of two parts: The Definition (telling Python what the function does) and The Call (telling Python to run it right now).

Lesson Overview

In this lesson, we will cover:

• The def Keyword: How to start a function.
• The Body: Indentation and the logic inside.
• Calling the Function: Making it run.
• Execution Flow: Understanding how Python jumps around your code.

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1. Defining a Function

To define a function, we use the def keyword, followed by the name we want to give it, parentheses (), and a colon :.

def say_hello():
    # Everything indented here is part of the function
    print("Hello from inside the function!")
    print("Hope you're having a great day!")

Naming Rules: Function names follow the same rules as variables (lowercase, underscores between words, e.g., calculate_tax).

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2. Calling a Function

Simply defining a function does nothing. If you run the code above, your screen will be blank. You have to "call" it by typing its name with parentheses.

# The Definition
def greet():
    print("Welcome to Python!")

# The Call
greet()

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3. The Flow of Execution

In a normal script, Python runs line 1, 2, 3... but with functions, it "jumps" around.

print("1. This is the first line.")

def my_func():
    print("3. This is inside the function.")

print("2. This is after the definition.")

my_func() # Python jumps back up to the definition here!

print("4. This is the final line.")

Output:

1. This is the first line.
2. This is after the definition.
3. This is inside the function.
4. This is the final line.

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Practice Exercise: The Function Script

Create a file named simple_functions.py.

1. Define a function named introduction that prints your name and your dream job.
2. Define a second function named current_goal that prints what you want to achieve by the end of this course.
3. Call both functions in order.
4. Call the introduction function three times in a row and notice how you didn't have to re-type the print statements!

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Quick Knowledge Check

1. What keyword is used to start a function definition?
2. Why doesn't the code inside a function run as soon as Python sees it?
3. How do you tell Python that a line of code belongs to a specific function?
4. Can you call a function before you have defined it (at the top of the file)?

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Key Takeaways

• def is used to define functions.
• Function names must be followed by ():.
• Indentation defines the "body" of the function.
• A function only executes when it is "called" by its name.

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What’s Next?

Our functions are currently static—they always do the exact same thing. But professional functions usually take data and give something back. In Lesson 3, we’ll explore Parameters and Return Values!