Let’s be honest. There’s a huge gap between writing code that works and writing code that’s actually good. It’s the number one thing that separates a junior developer from a senior, and it’s something a surprising number of us never really learn.

If you’re serious about your craft, you’ve probably felt this. You build something, it functions, but deep down you know it’s brittle. You’re afraid to touch it a year from now.

Today, we’re going to bridge that gap. I’m going to walk you through eight design principles that are the bedrock of professional, production-level code. This isn’t about fancy algorithms; it’s about a mindset. A way of thinking that prepares your code for the future.

And hey, if you want a cheat sheet with all these principles plus the code examples I’m referencing, you can get it for free. Just sign up for my newsletter from the link in the description, and I’ll send it right over.

Ready? Let’s dive in.

1. Cohesion & Single Responsibility

This sounds academic, but it’s simple: every piece of code should have one job, and one reason to change.

High cohesion means you group related things together. A function does one thing. A class has one core responsibility. A module contains related classes.

Think about a UserManager class. A junior dev might cram everything in there: validating user input, saving the user to the database, sending a welcome email, and logging the activity. At first glance, it looks fine. But what happens when you want to change your database? Or swap your email service? You have to rip apart this massive, god-like class. It’s a nightmare.

The senior approach? Break it up. You’d have:

• An EmailValidator class.
• A UserRespository class (just for database stuff).
• An EmailService class.
• A UserActivityLogger class.

Then, your main UserService class delegates the work to these other, specialized classes. Yes, it’s more files. It looks like overkill for a small project. I get it. But this is systems-level thinking. You’re anticipating future changes and making them easy. You can now swap out the database logic or the email provider without touching the core user service. That’s powerful.

2. Encapsulation & Abstraction

This is all about hiding the messy details. You want to expose the behavior of your code, not the raw data.

Imagine a simple BankAccount class. The naive way is to just have public attributes like balance and transactions. What could go wrong? Well, another developer (or you, on a Monday morning) could accidentally set the balance to a negative number. Or set the transactions list to a string. Chaos.

The solution is to protect your internal state. In Python, we use a leading underscore (e.g., _balance) as a signal: “Hey, this is internal. Please don’t touch it directly.”

Instead of letting people mess with the data, you provide methods: deposit(), withdraw(), get_balance(). Inside these methods, you can add protective logic. The deposit() method can check for negative amounts. The withdraw() method can check for sufficient funds.

The user of your class doesn’t need to know how it all works inside. They just need to know they can call deposit(), and it will just work. You’ve hidden the complexity and provided a simple, safe interface.

3. Loose Coupling & Modularity

Coupling is how tightly connected your code components are. You want them to be as loosely coupled as possible. A change in one part shouldn’t send a ripple effect of breakages across the entire system.

Let’s go back to that email example. A tightly coupled OrderProcessor might create an instance of EmailSender directly inside itself. Now, that OrderProcessor is forever tied to that specific EmailSender class. What if you want to send an SMS instead? You have to change the OrderProcessor code.

The loosely coupled way is to rely on an “interface,” or what Python calls an Abstract Base Class (ABC). You define a generic Notifier class that says, “Anything that wants to be a notifier must have a send() method.”

Then, your OrderProcessor just asks for a Notifier object. It doesn’t care if it’s an EmailNotifier or an SmsNotifier or a CarrierPigeonNotifier. As long as the object you give it has a send() method, it will work. You’ve decoupled the OrderProcessor from the specific implementation of the notification. You can swap them in and out interchangeably.

---

A quick pause. I want to thank boot.dev for sponsoring this discussion. It’s an online platform for backend development that’s way more interactive than just watching videos. You learn Python and Go by building real projects, right in your browser. It’s gamified, so you level up and unlock content, which is surprisingly addictive. The core content is free, and with the code techwithtim, you get 25% off the annual plan. It’s a great way to put these principles into practice. Now, back to it. —

4. Reusability & Extensibility

This one’s a question you should always ask yourself: Can I add new functionality without editing existing code?

Think of a ReportGenerator function that has a giant if/elif/else block to handle different formats: if format == 'text', elif format == 'csv', elif format == 'html'. To add a JSON format, you have to go in and add another elif. This is not extensible.

The better way is, again, to use an abstract class. Create a ReportFormatter interface with a format() method. Then create separate classes: TextFormatter, CsvFormatter, HtmlFormatter, each with their own format() logic.

Your ReportGenerator now just takes any ReportFormatter object and calls its format() method. Want to add JSON support? You just create a new JsonFormatter class. You don’t have to touch the ReportGenerator at all. It’s extensible without being modified.

5. Portability

This is the one everyone forgets. Will your code work on a different machine? On Linux instead of Windows? Without some weird version of C++ installed?

The most common mistake I see is hardcoding file paths. If you write C:\Users\Ahmed\data\input.txt, that code is now guaranteed to fail on every other computer in the world.

The solution is to use libraries like Python’s os and pathlib to build paths dynamically. And for things like API keys, database URLs, and other environment-specific settings, use environment variables. Don’t hardcode them! Create a .env file and load them at runtime. This makes your code portable and secure.

6. Defensibility

Write your code as if an idiot is going to use it. Because someday, that idiot will be you.

This means validating all inputs. Sanitizing data. Setting safe default values. Ask yourself, “What’s the worst that could happen if someone provides bad input?” and then guard against it.

In a payment processor, don’t have debug_mode=True as the default. Don’t set the maximum retries to 100. Don’t forget a timeout. These are unsafe defaults.

And for the love of all that is holy, validate your inputs! Don’t just assume the amount is a number or that the account_number is valid. Check it. Raise clear errors if it’s wrong. Protect your system from bad data.

7. Maintainability & Testability

The most expensive part of software isn’t writing it; it’s maintaining it. And you can’t maintain what you can’t test.

Code that is easy to test is, by default, more maintainable.

Look at a complex calculate function that parses an expression, performs the math, handles errors, and writes to a log file all at once. How do you even begin to test that? There are a million edge cases.

The answer is to break it down. Have a separate OperationParser. Have simple add, subtract, multiply functions. Each of these small, pure components is incredibly easy to test. Your main calculate function then becomes a simple coordinator of these tested components.

8. Simplicity (KISS, DRY, YAGNI)

Finally, after all that, the highest goal is simplicity.

• KISS (Keep It Simple, Stupid): Simple code is harder to write than complex code, but it’s a million times easier to understand and maintain. Swallow your ego and write the simplest thing that works.
• DRY (Don’t Repeat Yourself): If you’re doing something more than once, wrap it in a reusable function or component.
• YAGNI (You Aren’t Gonna Need It): This is the counter-balance to all the principles above. Don’t over-engineer. Don’t add a flexible, extensible system if you’re just building a quick prototype to validate an idea. When I was coding my startup, I ignored a lot of these patterns at first because speed was more important. Always ask what the business need is before you start engineering a masterpiece.

Phew, that was a lot. But these patterns are what it takes to level up. It’s a shift from just getting things done to building things that last.

If you enjoyed this, let me know. I’d love to make more advanced videos like this one. See you in the next one.