Logs, Stack Traces & Error Reports

Master the error output and you master debugging.

Stack Traces

Show the path that led to error:

File "main.py", line 15, in <module>
    process_user(user_id)
    ↓
File "service.py", line 42, in process_user
    user = get_user(user_id)
    ↓
File "db.py", line 8, in get_user
    return db.query(User).filter(id=user_id).first()
    ↓
AttributeError: 'NoneType' object has no attribute 'first'

Read backwards:

Last line: The actual error
Lines above: How you got there

Log Levels

DEBUG:   Detailed info for debugging
INFO:    General information (startup, etc)
WARNING: Something unexpected, not an error
ERROR:   Something failed, but app continues
CRITICAL: App is stopping

Use appropriate level for clarity.

Error Messages

Good error messages have:

❌ Bad:
"Error 404"

✅ Good:
"User with ID 123 not found in database. 
 Expected user to exist before processing. 
 Check if user was created successfully."

Reporting Errors

When reporting a bug, include:

Exact error message
What you did to trigger it
Expected vs actual behavior
Stack trace (if available)
Environment (OS, Python version, etc)

The error message is your best friend. Read it carefully before Googling.

CI Failures Overview