7 Python Design Flaws I See in 90% of Code Reviews

I’ve been reviewing Python code for over four years now — from fresh bootcamp projects to production systems running across thousands of servers. And I can tell you this: most developers, even the experienced ones, keep repeating the same design mistakes over and over.

These aren’t syntax errors or rookie bugs. These are subtle design flaws — the kind that make codebases brittle, unreadable, and impossible to scale.

Let’s talk about them.

1. Using Functions for Everything (When Classes Would Save You)

You know that one developer who insists that “functions are simpler”? Sure, until they’ve got 12 helper functions, all sharing the same parameters, passing around state like a game of hot potato.

# Common anti-pattern
def calculate_discount(price, discount_rate, tax_rate):
    discounted = price - (price * discount_rate)
    return discounted + (discounted * tax_rate)
def process_order(order, discount_rate, tax_rate):
    total = 0
    for item in order['items']:
        total += calculate_discount(item['price'], discount_rate, tax_rate)
    return total