Separation of Concerns

One of the most important principles in software development is separation of concerns. The idea is simple: each piece of code should handle one responsibility and handle it well. When you mix multiple responsibilities together, you create code that's hard to understand, modify, and debug.

What Is a Concern?

A "concern" is a distinct responsibility or aspect of your program. Common concerns include:

Data access: Getting and storing information
Business logic: Processing and transforming data
Presentation: Displaying information to users
Validation: Checking that inputs are correct

When these concerns get tangled together, changing one thing risks breaking another.

Tangled vs Separated Code

Here's code that mixes concerns — it fetches data, processes it, and displays it all in one place:

def do_everything(ticker):
    response = requests.get(f"api/{ticker}")
    data = response.json()
    price = data['price']
    print(f"${price:.2f}")

This seems simple, but what happens when you need to change how prices display? You're editing the same function that handles network requests. What if you want to test the price formatting without making real API calls? You can't.

Here's the same functionality with separated concerns:

def fetch_stock_data(ticker):
    response = requests.get(f"api/{ticker}")
    return response.json()

def extract_price(data):
    return data['price']

def display_price(ticker, price):
    print(f"{ticker}: ${price:.2f}")

Now each function does exactly one thing. You can modify display formatting without touching network code. You can test price extraction with fake data.

Why AI Loves Separation

When you ask AI to modify tangled code, it struggles to make surgical changes. It might accidentally break the data fetching while trying to fix the display.

With separated code, you can give AI one small, focused module. "Here's my 30-line data fetching module. Add retry logic." AI can focus entirely on that concern without worrying about unrelated code.

Structure your prompts to encourage separation: "Create separate functions for fetching data, processing results, and displaying output."

The Practical Benefit

Separated code is easier for both humans and AI to work with. Each piece can be understood in isolation, tested independently, and modified without fear. When something breaks, you know exactly where to look.

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