Name That Error

A traceback goes on the board with no program attached. Same two questions as last year — what does Python mean, and what kind of line caused it — plus a third that only matters now: whose code is broken? Every traceback in this course comes from a program with more than one file, and the frame that crashed is very often not the frame that is wrong.

The board today

Traceback (most recent call last):
  File "/home/student/main.py", line 4, in <module>
    weekly_summary(weeks_sessions)
  File "/home/student/report.py", line 6, in weekly_summary
    average = average_attendance(sessions)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/student/attendance.py", line 6, in average_attendance
    return total / len(sessions)
           ~~~~~~^~~~~~~~~~~~~~~
ZeroDivisionError: division by zero

Three frames, three files, and only the top one belongs to whoever is running the program. Read it bottom to top and the story is: a division by zero happened inside average_attendance, which was called by weekly_summary, which was called from line 4 of main.py.

The crash site is not the crime scene

attendance.py divided by zero, but attendance.py is not necessarily wrong. Somebody handed it an empty list. The bug is either in main.py for asking about a week with no sessions, or in attendance.py for not saying what it does with an empty one — and that is the argument worth having in a code review.

Which frame do you go to first?

flowchart TD
    A["A traceback appeared"] --> B{"Read the bottom line:<br/>what went wrong?"}
    B --> C{"Which frames are in<br/>files you wrote?"}
    C -->|"the top one only"| D["Start there:<br/>what did you pass in?"]
    C -->|"none of them"| E["Find the last frame<br/>you can change"]
    C -->|"a middle one"| F["Start at the lowest<br/>frame you own"]
    D --> G["Reproduce it with<br/>the smallest input"]
    E --> G
    F --> G

The move that saves the most time is the last one: shrink the input until the crash still happens. An empty list is usually as small as it gets, and an empty list is usually the answer.

The vocabulary, extended

Grade 11’s five errors still turn up. These are the ones that arrive once you are using somebody else’s classes and modules.

Message	What it tells you about the code you did not write
AttributeError: 'Session' object has no attribute 'spaces_remaining'	The method exists under another name — check the class definition, not your memory
AttributeError: 'NoneType' object has no attribute 'name'	A function returned None on failure and nobody checked before using the result
TypeError: Session.__init__() missing 1 required positional argument: 'capacity'	You called their constructor with fewer arguments than it requires
KeyError: 'Wednesday'	The dictionary has no such key; the caller assumed a key that the data does not guarantee
RecursionError: maximum recursion depth exceeded	A base case was never reachable from the argument passed in

The middle row is the one that hides. None travels: a lookup fails quietly, the None gets stored, and the crash happens three functions later in a file that is entirely innocent.

How to run it

1. One traceback on the board, no context.
2. Everyone writes the diagnosis in one sentence, then names the frame they would open first — and says why.
3. Compare. Then reveal the program and check.

The full method is Reading a Traceback in Someone Else’s Code; this warm-up is the drill that makes it fast.

Curriculum connection

A4.1

work independently, using support documentation (e.g., IDE Help, tutorials, websites, user manuals), to resolve syntax issues during software development;

Link to original