Ahmed Bouchefra
Let’s be honest. There’s a huge gap between writing code thatworksand writing code that’s actuallygood. 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 aUserManagerclass. 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
EmailValidatorclass. - A
UserRespositoryclass (just for database stuff). - An
EmailServiceclass. - A
UserActivityLoggerclass.
Then, your mainUserServiceclassdelegatesthe 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 toexpose the behavior of your code, not the raw data.
Imagine a simpleBankAccountclass. The naive way is to just have public attributes likebalanceandtransactions. What could go wrong? Well, another developer (or you, on a Monday morning) could accidentally set the balance to a negative number. Or set thetransactionslist 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. Thedeposit()method can check for negative amounts. Thewithdraw()method can check for sufficient funds.
The user of your class doesn’t need to knowhowit all works inside. They just need to know they can calldeposit(), 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 coupledOrderProcessormight create an instance ofEmailSenderdirectly inside itself. Now, thatOrderProcessoris forever tied to thatspecificEmailSenderclass. What if you want to send an SMS instead? You have to change theOrderProcessorcode.
The loosely coupled way is to rely on an “interface,” or what Python calls an Abstract Base Class (ABC). You define a genericNotifierclass that says, “Anything that wants to be a notifiermusthave asend()method.”
Then, yourOrderProcessorjust asks for aNotifierobject. It doesn’t care if it’s anEmailNotifieror anSmsNotifieror aCarrierPigeonNotifier. As long as the object you give it has asend()method, it will work. You’ve decoupled theOrderProcessorfrom the specific implementation of the notification. You can swap them in and out interchangeably.
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4. Reusability & Extensibility
This one’s a question you should always ask yourself:Can I add new functionality without editing existing code?
Think of aReportGeneratorfunction that has a giantif/elif/elseblock 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 anotherelif. This is not extensible.
The better way is, again, to use an abstract class. Create aReportFormatterinterface with aformat()method. Then create separate classes:TextFormatter,CsvFormatter,HtmlFormatter, each with their ownformat()logic.
YourReportGeneratornow just takes anyReportFormatterobject and calls itsformat()method. Want to add JSON support? You just create a newJsonFormatterclass. You don’t have to touch theReportGeneratorat 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 writeC:\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’sosandpathlibto 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.envfile 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 havedebug_mode=Trueas 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 theamountis a number or that theaccount_numberis 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’smaintainingit. And you can’t maintain what you can’t test.
Code that is easy to test is, by default, more maintainable.
Look at a complexcalculatefunction 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 separateOperationParser. Have simpleadd,subtract,multiplyfunctions. Each of these small, pure components is incredibly easy to test. Your maincalculatefunction 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.
