How to Read a Traceback

A traceback is Python’s report of where execution was when the program failed. It can look noisy, but the first useful pass is simple:

• start near the bottom;
• find the error type;
• read the message;
• read the line of your code mentioned just above it.

Here is a small program:

It fails because one value is a string:

TypeError: unsupported operand type(s) for +: 'int' and 'str'

The error type is TypeError. The message says Python tried to add an integer and a string. The failing line is inside the loop:

total = total + value

That line assumed value was a number.

The extra print is not the final fix. It is a temporary inspection. It shows which value was being handled when the failure occurred.

Tracebacks are stacked calls

If one function calls another function, the traceback may show several frames. A frame is one active function call. The bottom frame is often closest to the actual error.

The traceback tells a story:

mean called total
total failed while adding a value

Read it as a path, not as a wall of text.

What not to do

Do not copy the whole traceback into your head. You usually need only four pieces: