Python Functions: Define, Call, and Return Values

Imagine you have a favorite recipe for pancakes. Every Sunday morning, you follow the same steps: mix flour, crack eggs, pour batter, flip. You don't reinvent the recipe each time — you just follow the one you already know.

Functions in Python work exactly like recipes. You write the instructions once, give them a name, and then call that name whenever you need those instructions to run again. Write once, use anywhere.

In this tutorial, you'll learn how to create your own functions, pass data into them, and get results back. By the end, you'll be writing clean, reusable code like a real developer.

To create a function in Python, you use the def keyword, followed by a name, parentheses, and a colon. Everything indented underneath belongs to that function.

If you run the code above, nothing happens. That's because defining a function is like writing down a recipe. It doesn't cook anything until you actually follow the steps.

To actually run the function, you call it by writing its name followed by parentheses:

Now you see the output. The parentheses () after greet are what tell Python to actually run the function. Without them, Python just looks at the recipe without cooking.

You can call a function as many times as you want. Each call runs all the code inside the function from top to bottom.

Our greet() function always says the same thing. That's like a recipe that only makes plain pancakes. What if you want blueberry pancakes? You need a way to pass in ingredients.

In Python, you pass data into a function using parameters. A parameter is a variable name listed inside the parentheses of the function definition. When you call the function, the actual value you send in is called an argument.

Here, name is the parameter — a placeholder. When we call greet('Alice'), the string 'Alice' is the argument — the actual value that fills the placeholder.

So far, our functions just print things. But most of the time, you want a function to calculate something and hand the result back to you. That's what return does.

Think of it this way: print() is like shouting the answer out loud. return is like writing the answer on a piece of paper and handing it to whoever asked.

The return statement does two things: it sends a value back to wherever the function was called, and it immediately exits the function. Any code after return inside the same function will not run.

Functions can accept as many parameters as you need. Just separate them with commas. When calling, the arguments are matched to parameters in order — the first argument goes to the first parameter, and so on.

The order matters. If you swap the arguments around, you'll get nonsense results or an error. introduce(25, 'Alice', 'London') would try to use 25 as a name.

Sometimes you want a parameter to have a fallback value — a sensible default that's used when the caller doesn't provide one. You set this with an = sign in the definition.

When we call greet('Alice'), the greeting parameter uses its default value 'Hello'. When we call greet('Bob', 'Hey'), we override the default with 'Hey'.

Absolutely! In fact, this is one of the most powerful ideas in programming. You can build small, focused functions and then combine them like building blocks.

Here, sum_of_squares calls square twice. Each small function does one simple job. Together, they solve a bigger problem. This approach makes your code easier to read, test, and fix.