Reading Error Messages
Actually reading the error: the first and most skipped debugging step.
Also known as: error messages, reading errors, understanding errors
The most common debugging mistake is not reading the error carefully. Most error messages tell you what went wrong and where. Skipping them to try random fixes wastes time.
Read it in this order
- The error type and message.
KeyError: 'price',TypeError: x is not a function. The type narrows down the kind of problem; the message names the specific value. - The location. File, line and column of the failing line, usually in the stack trace. Find the first line that is your code.
- The code at that line. What values are involved? Which one could be
None, missing, or the wrong type? - The context. What was the program doing? What input triggered it?
TypeError: Cannot read properties of undefined (reading 'name')
at renderUser (profile.js:12:25)Translation: on line 12, something called .name on a value that was undefined. So
the thing before .name is missing. Now you know exactly what to check.
Common patterns
| Message says | Usually means |
|---|---|
“is not defined” / NameError | A typo, or the variable isn’t in scope or isn’t imported |
| “undefined / None has no attribute” | A value you expected to exist doesn’t |
“unexpected token” / SyntaxError | A missing bracket, quote or comma, often before the reported line |
| “connection refused” / “timeout” | The other service isn’t reachable or running |
| “permission denied” / 403 | Authentication or access problem |
Habits
- Read the whole message, including the lines under it.
- Don’t fix the symptom. Find out why the value is missing.
- Search the exact message (without your own names) if it’s unfamiliar. Others have hit it.
- Include the full error when you ask for help, plus what you’ve tried.
If the message is unclear, see debugging for the next steps.