Writing scripts is one thing. Shipping production code? That’s a different game.

7 Python Mistakes I Stopped Making After Writing Production Code

These weren’t syntax errors — they were habits that made my code fragile, slow, and hard to maintain.

Photo by Sander Sammy on Unsplash

When I first started learning Python, I thought I had it all figured out. The syntax was clean, the community was massive, and the tutorials made everything seem easy. I built scripts, small apps, and played around with popular libraries. But then I started working on production code — and reality hit like a KeyError at runtime.

Production Python is a different beast. It’s not about making it work; it’s about making it reliable, readable, and maintainable. Through many code reviews, bug fixes, and late-night debugging sessions, I began noticing a pattern of mistakes I was making.

Here are 7 Python mistakes I stopped making after writing production code — and why you should too.

1. Using Mutable Default Arguments

The mistake:

def append_item(item, items=[]):
    items.append(item)
    return items