Python Day #7: The Step-by-Step Blueprint for Writing Python Functions
Learn how to write clean, reusable Python functions with type hints for error-free, easy-to-read code—perfect for beginners and pros alike.
Have you ever felt like you’re writing the same lines of code over and over? Let’s fix that.
Functions let you group code that does one thing into a little package you can use again and again. This saves time, helps you avoid mistakes, and makes your code easier to read and work with.
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In this lesson, we are going to begin the journey into reusable code. This is a core Python fundamental that you will inevitably use in your careers, functions. So breaking this down to a level you can understand is important to me.
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Functions aren’t just about cutting down on repetitive code. They help you keep your code clean, organized, and reusable. Every function you write should do one specific thing. When you add type hints, you make your code easier for you and anyone else reading it to understand.
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In this article, we’re going to build on what you’ve already learned. Last time, you worked on controlling the flow of your code. Now, it’s time to define your own functions and start writing clean, reusable code that makes you a better programmer.
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Reusable Blocks of Code
Congratulations, at this point you have a good understanding of the basics of programming and how you can mange the flow of your code. It’s time to take things to the next level, this concept will build perfectly on everything so far.
So far, you’ve worked with different built-in functions in Python. Can you name me a few? These are pieces of code, that have already been programmed to complete a certain task. All you need to do it call them and give them an argument.
In case you couldn’t name any, which shame on you if you couldn’t, you worked with most of these: print(), len(), input(), str(), int(), range(), float(). These are just to name a few, but these functions all serve a specific task.
The real questions are, how do they work? What is going on behind the scenes? Can I make my own function? Yes, you can make functions of your own. This is an important concept to learn now as everything will continue to build from here.
Essentially a function is just a reusable block of code that you define one time, then you can use it to complete a certain tasks anywhere in your program. If you’re writing code, and you know that the code you’re writing can be used more than once, you should store it in a function block.
The goal of writing a function is to make your code reusable, so you don’t have to write it all again. Every function you define should serve one specific purpose. Not two, not three, one. This will make your code clearer and you’ll be able to reuse that function in more scenarios.
Once you define a function of your own, you can use it just like you would any of those other Python functions we talked about earlier. You need to call it, and give it any needed arguments.
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How to Define a Function in Python
We need to set the stage for how you can create your own functions, but throw the word “create” away as the word we want to use is “define”. You need to define a function, that is what you are doing.
In Python we have the keyword def, this just means define and we use this at the start when we are going to define/declare a new function in our code.
After def, we name the function, that’s then followed by a set of parentheses. You can name you function anything, but the same naming conventions apply that you used for variables. No capitalization, no spaces, don’t start with numbers, etc.
Inside the parentheses you can put any parameters that the function will use. A parameter is just a variable that you will use in the function. The value to the parameters are arguments. We give arguments when you call the function.
Indentation is also very important here, as the next line should be indented to tell Python that this is the code that’s wrapped in the function. Let’s take a look at a basic set up.
Above you can see I defined my own function, check_even_numbers. I then gave this two parameters, num1 and num2. Currently these parameters have no value, but we can write the function as if they already do, think ahead.
All this function does is check if added together, these parameters are an even or odd number, then based on this it will return True or False. The key thing here is to remember a function returns a value.
When a function returns a value, we can store that value in a variable. So the value to the variable results is either going to be True or False, which ever the function returns to us.
We can then use that variable results in other parts of our code as it now has a value. Yes, you can have your function print off things, but you can not use a printed value, this is just for you.
A function should most of the time return a value that you can then use at another part in your code.
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Increase Code Readability
Okay, now is a get time to introduce a new Python topic that will make you a better developer. You will thank me and yourself later as this increase the readability of your code so at a quick glance you can see what a function is doing.
This new topic is called Type Hintings. These are basically additional hints and text you give to your code so you know exactly what type of data a function or variable accepts as well as what type of data a function returns.
Here is some new code that I’ve optimized for this purpose. You can see I start with three variables, then I can set the value to the variable so Python and I know what type of data should be the value.
This should also be done when we define a function, you can now see that I have three parameters, I have also set the data type value for them. If the function gets any other type of data than the type I specified we’d get an error.
Another point is we can use the → after the parameters, this says the type of data that the function will return in the end. Now when you read the function you know exactly the type of data it accepts and the type of data it will output!
This is one of the best habits you can pick up right now! You should start writing not only your functions but all your code with type hintings. You can see how much clearer everything becomes.
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Wrapping Up Lesson Seven
Alright, so now you know how to take that next big step in your coding journey—making and using your own functions.
Functions aren’t just there to avoid repeating the same code over and over. They help keep your code clean, organized, and easy to read, which makes it a lot simpler to understand and fix later on.
When you add type hints to your functions, you make it even easier to know what kind of stuff your code is expecting and what it gives back. Every function you write should really just do one thing, so it’s clear and simple.
Keep practicing writing your own functions with type hints—it’s one of the best things you can do to get better at coding. Best of luck nerds, you got this!
Let’s keep moving forward with Day #8 next week.
Hope you all have an amazing week nerds ~ Josh (Chief Nerd Officer 🤓)
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