8 Years as a Python Developer — The 10 Mistakes I See Junior Developers Make Every Single Day.

I’ve been writing Python professionally for 8 years. I’ve worked at startups, enterprises, healthcare companies, and big four consulting firms. I’ve reviewed hundreds of pull requests and mentored dozens of developers.

These are the 10 mistakes I see over and over again.

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1. Putting All Logic in Route Handlers

The most common mistake. Junior developers dump everything — database queries, business logic, validation, error handling — directly in the FastAPI or Django view function.

Bad pattern: @app.post(“/orders”) def create_order(data, db):

80 lines of mixed logic here

Good pattern: @app.post(“/orders”) def create_order(data, db): return order_service.create(data, db) # 1 line

Your route handler should only handle HTTP. Period.

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2. Not Using Environment Variables

I’ve seen production API keys committed to GitHub more times than I can count. Use python-dotenv or pydantic-settings. Never hardcode secrets.

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3. Ignoring Database Indexing

Writing a query that works fine with 100 rows and calling it done. Then wondering why the application crawls at 1 million rows.

Add indexes to columns you filter and sort by. It’s one line in SQLAlchemy.

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4. No Tests Until “Later”

Later never comes. Write tests as you build. I aim for 80%+ coverage on all backend services. PyTest with FastAPI’s TestClient makes this fast.

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5. Using print() Instead of Logging

print() disappears in production. Use Python’s logging module from day one. It costs nothing and saves hours of debugging.

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6. Not Handling Errors Properly

return None when something fails is not error handling. Use HTTPException with proper status codes. Your API consumers deserve meaningful error messages.

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7. Storing Passwords in Plain Text

This still happens. Use passlib with bcrypt. It’s a two-line change that protects your users.

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8. Not Documenting APIs

FastAPI gives you free Swagger docs. Use them. Write descriptions for your endpoints. Future you and your teammates will be grateful.

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9. Over-Engineering Early

Microservices, message queues, and distributed systems are great — when you actually need them. Start with a monolith. Refactor when the pain point is real, not imaginary.

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10. Not Using Virtual Environments

Every project gets its own virtual environment. Every single one. Dependency conflicts between projects are a preventable waste of time.

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Final Thought

Every senior developer you admire made every one of these mistakes. The difference is they learned from them and built habits around avoiding them. These aren’t failures — they’re the curriculum.

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