traceback

Learn stack traces with the traceback module.

The traceback module

To see the full call stack of an error you use the traceback module. You import it with import traceback and get the stack trace at the error moment as a string with traceback.format_exc().

This string shows which file and line the error started on; it lists each call frame in order. It's useful for error logging in production applications; not shown to the user but tells the developer a lot.

Exercise Print the first line of traceback.format_exc

Start with import traceback. In a try block call x = 1 / 0. In except ZeroDivisionError: get the stack trace with tb = traceback.format_exc(). Print the first line with print(tb.splitlines()[0]). The first line is "Traceback (most recent call last):".

Show example solution

import traceback

# Call 1 / 0 in a try block
try:
    x = 1 / 0
# In except ZeroDivisionError: get the trace with format_exc
except ZeroDivisionError:
    tb = traceback.format_exc()
    # Print the first line
    print(tb.splitlines()[0])

Reading a stack trace

A stack trace has three parts: the header (Traceback (most recent call last):), the call frames (file name, line, code) and the final line with the error type and message. The bottom line tells the actual error.

Frames show calls from top to bottom in aging order; the bottom frame is where the error happened. When debugging you start at the bottom and go up; this makes finding the source easier.

Test yourself

What does traceback.format_exc() return?
- Only the line number that raised
- Only the error type
- The error stack trace as a string
- A file object
Show answer

The error stack trace as a string — The string contains file, line and type info.

Complete the format_exc call

import traceback
tb = traceback.____()

Show answer

import traceback
tb = traceback.format_exc()

Match the traceback concepts

format_exc

Returns the trace as a string
import traceback

Imports the module
splitlines

Splits into lines
ZeroDivisionError

Division by zero error

Next lesson: Summary - Advanced exceptions

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