Debugging Common Errors in Full Stack Python Development

 

Debugging is a crucial part of full stack Python development, especially when working across both backend and frontend layers. Here’s a breakdown of common errors encountered in full stack Python development and how to debug them effectively.

🔧 Common Backend (Python/Django/Flask/FastAPI) Errors

1. Import Errors

Example: ModuleNotFoundError: No module named 'xyz'
Fix:

  • Ensure the package is installed (pip install xyz)

  • Check virtual environment activation

  • Watch for circular imports

2. Database Connection Issues

Example: OperationalError: could not connect to server
Fix:

  • Verify DATABASE_URL or settings in settings.py

  • Ensure the DB server is running (PostgreSQL, MySQL, etc.)

  • Use tools like psql or mysql CLI to test connection

3. Migrations and ORM Issues

Example: django.db.utils.ProgrammingError: relation "table_name" does not exist
Fix:

  • Run python manage.py makemigrations and migrate

  • Check for missing __init__.py in migration folders

  • Inspect model changes carefully when reverting migrations

4. Serialization/Validation Errors (Django REST Framework or Pydantic)

Example: KeyError, ValidationError, TypeError
Fix:

  • Validate data types passed to serializers

  • Use .get() instead of direct key access in dictionaries

  • Print/log input data to understand structure

5. CORS Errors

Example: Access to fetch at 'http://localhost:8000/api' from origin 'http://localhost:3000' has been blocked by CORS policy
Fix:

  • Add CORS headers using middleware (django-cors-headers for Django, flask-cors for Flask)

  • Configure frontend to match backend CORS settings

🌐 Common Frontend Errors (React/Vue/HTML + JS with Python Backend)

1. Incorrect API URLs

Fix:

  • Double-check your fetch/axios URLs and HTTP methods

  • Watch out for trailing slashes (Django REST Framework cares!)

2. JSON Parsing Errors

Example: SyntaxError: Unexpected token < in JSON
Fix:

  • Backend might be sending HTML error instead of JSON

  • Inspect the response using browser dev tools or Postman

3. 404/500 Errors from Backend

Fix:

  • Use browser dev tools (Network tab) to view request details

  • Check backend logs (print, logging, or tools like Sentry)

⚙️ Full Stack Integration Errors

1. Mismatch Between Frontend and Backend Data Structures

Fix:

  • Confirm expected request/response shape

  • Use tools like Swagger/OpenAPI or Postman collections

2. Async/Await Confusion in FastAPI

Fix:

  • Use async def only if calling async functions

  • Avoid mixing sync/async unless necessary

🛠️ Debugging Tools and Tips

  • Browser Dev Tools – inspect API requests, console errors

  • Postman / Insomnia – test APIs independently

  • Python Debuggers – use pdb, breakpoint(), or IDE-integrated debugging

  • Logging – use Python’s logging module, not just print()

  • Linting/Static Analysis – flake8, pylint, black, mypy help catch issues early

  • Error Monitoring – Sentry, Rollbar for production

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