Designing for Growth: Why SOLID Principles Matter in Large Systems

When software projects are small, it’s easy to write code that “just works.” But as systems scale - with more features, more developers, and more users - that early code can become a nightmare to maintain.

That’s why design principles matter. In particular, the SOLID principles act as a compass for object-oriented design. They guide developers to write software that is:

• Modular and reusable 🔁
• Easy to test 🔬
• Easier to read and change 👁️
• Resistant to bugs when new features are added 🐛❌

And perhaps most importantly: SOLID makes it easier to think clearly as your system grows.

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🤔 Why You Should Care About SOLID

Imagine you’re building a vehicle management system. In a small system, maybe a function just returns a string like "Car started".

But in a real-world, large-scale system, you don’t return strings. You return objects:

def start_engine(vehicle: IVehicle) -> EngineStatus:
    return EngineStatus(code=200, message='Engine started', fuel_level=72)

These objects encapsulate state and behavior. And the more complex your objects become, the more they depend on good architecture to stay clean and testable.

That’s where SOLID comes in.

🔣 The SOLID Principles Explained

Let’s explore the five SOLID principles through the metaphor of building a car 🚗 - a system most people intuitively understand.

S - Single Responsibility Principle (SRP)

“A class should have one, and only one, reason to change.” - Robert C. Martin

Each class or module should focus on one job.

🛑 Bad Example:

class Car:
    def drive(self): ...
    def play_music(self): ...
    def log_trip_data(self): ...
    def calculate_fuel_stats(self): ...

This Car class is doing too much. If any of those functionalities change, the entire class might need to be rewritten.

✅ Good Example:

class Car:
    def drive(self): ...

class MusicSystem:
    def play(self): ...

class TripLogger:
    def log(self): ...

Each class handles one responsibility. That means a change in the music system won’t accidentally break the driving logic. It also means multiple developers can work on different parts of the code without stepping on each other.

O - Open/Closed Principle (OCP)

“Software entities should be open for extension but closed for modification.”

Your code should allow new features to be added without changing the old, working code.

🛑 Bad Example:

class Car:
    def drive(self):
        ...
    def self_drive(self):  # added new feature directly
        ...

Now every time you want to upgrade the self-driving logic, you’re editing the main Car class-risking bugs in the existing driving logic.

✅ Good Example:

class SelfDrivingFeature:
    def assist(self, car: Car): ...

You can add new behavior by extending, not modifying existing classes. This makes your system stable and flexible.

Real-world impact? In large teams, your teammates don’t have to worry that their changes will break your code.

L - Liskov Substitution Principle (LSP)

“Subtypes must be substitutable for their base types.”

If Car is a parent class, then any subclass like ElectricCar should be usable anywhere a Car is expected-without surprises.

🛑 Bad Example:

class Car:
    def refuel(self): ...

class ElectricCar(Car):
    def refuel(self):
        raise NotImplementedError("Electric cars don't refuel.")

This breaks the contract. Code expecting a Car will crash if it gets an ElectricCar.

✅ Good Example:

Use better abstraction:

class FuelCar:
    def refuel(self): ...

class ElectricCar:
    def recharge(self): ...

Then use interfaces:

class IVehicle:
    def start(self): ...
class ElectricCar(IVehicle): ...
class FuelCar(IVehicle): ...

Now both car types are interchangeable under a common interface, but only implement what they need. You keep your system robust and predictable.

I - Interface Segregation Principle (ISP)

“Clients should not be forced to depend on methods they do not use.”

Large interfaces are tempting-but dangerous. They make classes implement things they don’t need.

🛑 Bad Example:

class ICarFeatures:
    def play_cd(self): ...
    def open_sunroof(self): ...
    def enable_4wd(self): ...

Now even a basic compact car has to implement enable_4wd() - even if it doesn’t support it.

✅ Good Example:

Break large interfaces into smaller, focused ones:

class IMusicPlayer:
    def play(self): ...

class ISunroofControl:
    def open(self): ...

class IAllWheelDrive:
    def enable(self): ...

Cars can now opt into only the features they support. Clean, lean, and easier to test.

D - Dependency Inversion Principle (DIP)

“Depend on abstractions, not concrete implementations.”

High-level modules (like your main Car) should not be tightly coupled to specific engines, sensors, or hardware. They should depend on interfaces, not concrete classes.

🛑 Bad Example:

class Car:
    def __init__(self):
        self.engine = GasEngine()

Now if you want to use an ElectricEngine, you have to edit the Car class.

✅ Good Example:

class Car:
    def __init__(self, engine: IEngine):
        self.engine = engine

Now you can pass in any engine:

car = Car(engine=ElectricEngine())

You’ve decoupled the engine from the car. This makes the system easier to test, reuse, and adapt.

Designing Objects That Scale

In real-world systems, we don’t just return strings like "OK" or codes like 404. We return objects that represent real domain concepts.

For example:

class EngineStatus:
    def __init__(self, running: bool, temperature: float, fuel_level: float):
        ...

This object carries state, behavior, and meaning - and is often passed around or extended. That’s why good object design becomes crucial at scale.

When your objects follow SOLID principles:

• They stay focused (SRP)
• They can evolve without breaking others (OCP, LSP)
• They implement only what’s necessary (ISP)
• And they’re flexible to test and extend (DIP)

Clean architecture isn’t just about individual lines of code - it’s about shaping how your objects talk to each other.

Easy Way to Remember SOLID

Think like a LEGO set:

• S: Separate the bricks (Single Responsibility)
• O: Add new pieces, don’t reshape old ones (Open/Closed)
• L: Any brick should fit where expected (Liskov Substitution)
• I: Don’t force all bricks to snap together (Interface Segregation)
• D: Use connectors, not glue (Dependency Inversion)

✨ Build systems like LEGO: modular, replaceable, and fun to scale.

🧪 Complete Example: All SOLID Principles Together

Here’s a small, end-to-end Python example that brings all five SOLID principles together using the car metaphor we’ve followed throughout this post.

from abc import ABC, abstractmethod
from dataclasses import dataclass

# I: Interface Segregation Principle (ISP)
class IEngine(ABC):
    @abstractmethod
    def start(self) -> str:
        pass

class IRefuelable(ABC):
    @abstractmethod
    def refuel(self) -> str:
        pass

class IRechargeable(ABC):
    @abstractmethod
    def recharge(self) -> str:
        pass

# D: Dependency Inversion Principle (DIP)
class ElectricEngine(IEngine, IRechargeable):
    def start(self) -> str:
        return "🔋 Electric engine started."

    def recharge(self) -> str:
        return "⚡ Battery recharged."

class GasEngine(IEngine, IRefuelable):
    def start(self) -> str:
        return "⛽ Gas engine started."

    def refuel(self) -> str:
        return "⛽ Tank refilled."

class HybridEngine(IEngine, IRefuelable, IRechargeable):
    def start(self) -> str:
        return "⚡⛽ Hybrid engine started."

    def refuel(self) -> str:
        return "⛽ Hybrid tank refilled."

    def recharge(self) -> str:
        return "⚡ Hybrid battery recharged."

class MockEngine(IEngine):
    def start(self) -> str:
        return "[TEST] Mock engine started."

# S: Single Responsibility Principle (SRP)
@dataclass
class TripLogger:
    def log_start(self):
        print("📝 Trip started.")

    def log_fuel(self, message: str):
        print(f"📋 Fuel Log: {message}")

    def log_charge(self, message: str):
        print(f"📋 Battery Log: {message}")

@dataclass
class FuelSystem:
    engine: IRefuelable
    logger: TripLogger

    def refuel(self):
        message = self.engine.refuel()
        self.logger.log_fuel(message)

@dataclass
class BatterySystem:
    engine: IRechargeable
    logger: TripLogger

    def recharge(self):
        message = self.engine.recharge()
        self.logger.log_charge(message)

# O: Open/Closed Principle (OCP)
@dataclass
class Car:
    engine: IEngine
    logger: TripLogger

    def drive(self):
        self.logger.log_start()
        print(self.engine.start())
        print("🚗 Car is driving...")

# ✅ Liskov Substitution Principle (LSP)
# All engines conform to IEngine and optional fuel/charge interfaces

def main():
    logger = TripLogger()

    cars = [
        ("Electric", Car(ElectricEngine(), logger)),
        ("Gas", Car(GasEngine(), logger)),
        ("Hybrid", Car(HybridEngine(), logger)),
        ("Mock", Car(MockEngine(), logger))
    ]

    for label, car in cars:
        print(f"\n=== {label} Car ===")
        car.drive()

        if isinstance(car.engine, IRechargeable):
            BatterySystem(car.engine, logger).recharge()

        if isinstance(car.engine, IRefuelable):
            FuelSystem(car.engine, logger).refuel()

if __name__ == "__main__":
    main()

# Output:
# === Electric Car ===
# 📝 Trip started.
# 🔋 Electric engine started.
# 🚗 Car is driving...
# 📋 Battery Log: ⚡ Battery recharged.

# === Gas Car ===
# 📝 Trip started.
# ⛽ Gas engine started.
# 🚗 Car is driving...
# 📋 Fuel Log: ⛽ Tank refilled.

# === Hybrid Car ===
# 📝 Trip started.
# ⚡⛽ Hybrid engine started.
# 🚗 Car is driving...
# 📋 Battery Log: ⚡ Hybrid battery recharged.
# 📋 Fuel Log: ⛽ Hybrid tank refilled.

# === Mock Car ===
# 📝 Trip started.
# [TEST] Mock engine started.
# 🚗 Car is driving...

✅ What this example shows:

Principle	In Action
SRP	TripLogger, BatterySystem, and FuelSystem each handle only one concern.
OCP	You can add SolarEngine() or BiofuelEngine() without touching Car.
LSP	Every IEngine subclass behaves correctly when used as a Car engine.
ISP	Clients like FuelSystem depend only on IRefuelable; not everything needs recharge().
DIP	Car, BatterySystem, and FuelSystem all depend on abstractions (IEngine, IRefuelable, IRechargeable).

🏁 Conclusion

The SOLID principles are more than just theory. They’re practical tools for building:

• Systems that scale with features
• Teams that scale with people
• Codebases that scale with time

Whether you’re building a car system, a payment processor, or a genome analysis engine - SOLID will keep you sane.