How to Read a Stack Trace

A stack trace is the trail of function calls that led to an error. It looks intimidating because it is long, but you only need two lines from it.

A Python traceback#

Traceback (most recent call last):
  File "app.py", line 20, in <module>
    report = build_report(orders)
  File "app.py", line 14, in build_report
    return summarise(items)
  File "app.py", line 8, in summarise
    average = total / len(items)
ZeroDivisionError: division by zero

Read it like this:

• Bottom line — the error type and message. Start here.
• Line directly above it — the exact line that failed: average = total / len(items).
• Lines above that — how the program got there, oldest at the top.

Python literally tells you the order: “most recent call last”. The bottom is the scene of the crash; the top is where the journey started.

Here, items was empty. The fix is not at line 8 — it is deciding what an empty order list should mean, probably at line 20.

A JavaScript stack trace#

TypeError: Cannot read properties of undefined (reading 'name')
    at showUser (app.js:12:20)
    at handleClick (app.js:5:3)
    at HTMLButtonElement.<anonymous> (app.js:2:24)

JavaScript is the other way round: the error is at the top, and the most recent call is on the first at line. Read downward to see how you arrived.

app.js:12:20 means file app.js, line 12, column 20.

Finding your code in a long trace#

When a library is involved, the trace can be forty lines. Scan for the first entry containing a file you wrote — that is almost always where the real problem is.

    at Array.map (<anonymous>)
    at renderList (app.js:34:18)        <-- your file, start here
    at node_modules/react-dom/...

Library code is usually correct. What is wrong is the data you handed it.

Chained errors in Python#

ValueError: invalid literal for int() with base 10: 'abc'

During handling of the above exception, another exception occurred:

KeyError: 'total'

Two errors: one happened while handling another. The first one is usually the real cause; the second is often a broken error handler. Fix the top one first.

You will also see “The above exception was the direct cause of the following exception”, which means the same thing — start at the top.

Making traces useful in your own code#

Do not swallow the trace:

try:
    process(data)
except Exception:
    print("Something went wrong")     # you have thrown away the evidence

import traceback
try:
    process(data)
except Exception:
    traceback.print_exc()             # keeps the full trace

In JavaScript, log the whole error object rather than just its message — console.error(error) keeps the stack, console.log(error.message) does not.

Minified traces#

In a production build you may see at t (main.a1b2.js:1:4820), which is useless. Source maps translate that back to your original files; most build tools produce them, and DevTools uses them automatically if they are available.

Where to go next#

• Debugging JavaScript in the browser
• Common Python errors