Recipe

Read a traceback

Use when: a Python program stopped and printed a report

Goal: the line to fix and what is wrong with it

Taught in Input, output, and first errors

1. 1

   Read the exception type and its message, and any Did you mean hint.

   Example

   NameError: name 'totl' is not defined. Did you mean: 'total'? says an unknown name, probably a typo for total.

   Why: the last line is the only one that says what went wrong; everything above says where.

2. 2

   Go up one or two lines to the file name and line number, and the code with ^ marks under the failing part.

   Example

   File "main.py", line 3, in <module> and print("Total:", totl) with ^^^^ under totl.

   Why: the marks point at the exact part of the line, not just the line.

3. 3

   No Traceback line and nothing printed: a grammar error, found before running. Otherwise it happened while running, at that line.

   • SyntaxError or IndentationError — fix the grammar; if the named line looks fine, check the line above it.
   • NameError or TypeError — look at the names and values used on that line.

   Why: grammar is checked for the whole file before any line runs; names and values only when the line runs.

4. 4

   Change only what the report points at, then rerun to see the next report, if there is one.

   Why: one change at a time tells you which change fixed it.