Programming Fundamentals › Error Handling

Stack Trace

The chain of function calls that led to an error.

Also known as: traceback, stack traces, call stack trace, backtrace

When something crashes, the program prints a stack trace: the list of function calls that were active at that moment, from where the error happened back to where it all started. It tells you where and how you got there.

Traceback (most recent call last):
  File "app.py", line 14, in <module>
    main()
  File "app.py", line 10, in main
    total = price_with_tax(item)
  File "app.py", line 5, in price_with_tax
    return item["price"] * 1.2
KeyError: 'price'

The last line is the error itself (KeyError: 'price'). The lines above are the calls that led to it.

How to read one

1. Start with the error message. What kind of error, and what does it say?
2. Find the first line that’s your code, not a library’s. Frameworks add many frames; the bug is usually in the first frame you wrote.
3. Open that file and line, then look at the values involved.
4. Use the frames above it to see how you got there, such as which caller passed the bad data.

Order differs between languages

Python prints the most recent call last. JavaScript (and Java) print the most recent call first, right under the error message. Check which one you’re looking at.

TypeError: Cannot read properties of undefined (reading 'price')
    at priceWithTax (app.js:5:20)     <- where it crashed
    at main (app.js:10:15)
    at app.js:14:1

Don’t paste only the last line when asking for help; the whole trace is the useful part. The skill of reading these is covered in reading error messages.