Common Beginner Coding Errors

Version: 0.17.0

When to Use

• Beginner reports error message
• Code isn't working as expected
• User asks "Why isn't this working?"

Error Diagnosis Process

1. Get complete error (exact message, file/line, what they tried)
2. Identify category (keyword-based)
3. Guide to solution with explanation
4. Teach prevention

Errors by Category

1. File Management

Changes Don't Appear:

Template Not Found:

• Flask: Templates must be in templates/ folder (exact name)
• Sinatra: Templates must be in views/ folder (exact name)

2. Python Errors

IndentationError:

• Python uses spaces for grouping (not { })
• Use 4 spaces per level, don't mix tabs/spaces
• Check all lines in block have same indent

ModuleNotFoundError: No module named 'flask':

3. Ruby/Sinatra Errors

uninitialized constant Sinatra:

1. Check: bundle list | grep sinatra
2. Verify Gemfile has gem 'sinatra'
3. Run: bundle install
4. Use: bundle exec ruby app.rb

4. Route/URL Errors

404 Not Found:

5. Server Errors

Address Already in Use:

1. Find other terminal with server running, Ctrl+C
2. Or kill process: lsof -i :5000 then kill <PID>
3. Or use different port: port=5001

6. Function Errors

Nothing Returned from Route: Routes must return something (HTML, template, redirect, JSON).

• Flask: return render_template('index.html', ...)
• Sinatra: erb :index

Debugging Mindset

1. Read error message (file, line number)
2. Form hypothesis
3. Make one small change
4. Test
5. Learn from it

End of Skill