Tracebacks & try/except

Not all bugs are equal. Python tells you which kind you have — learning to read that message is the skill.

The three error families

Syntax errors — Python cannot even start. It points at the line it chokes on:

if x > 5
    print("big")
# SyntaxError: expected ':'

Runtime errors (exceptions) — the program starts, then crashes mid-run:

ages = {"minh": 21}
print(ages["linh"])
# KeyError: 'linh'

Logic errors — the program runs to completion and gives the WRONG answer. No error message appears. These are the most dangerous, and the reason testing exists:

average = a + b / 2   # runs fine — and is wrong

The traceback: read it bottom-up

Traceback (most recent call last):
  File "report.py", line 4, in <module>
    total = sum(values) / count
ZeroDivisionError: division by zero

Read the LAST line first: the error type and message. Then the line above: where it happened. Then follow the call chain upward to find what led there.