Errors & testing

Three kinds of error

• A syntax error breaks the rules of Python, so the code will not run.
• A runtime error crashes the program while it is running.
• A logic error runs fine but gives the wrong answer.

Syntax errors

• These are typos: a missing :, an unclosed quote, a wrong indent.
• Python refuses to start and shows you the line.
• Fix the spelling or punctuation, then run again.

Runtime errors

• The code starts, then hits something it cannot do.
• Examples: 10 / 0, or int("hello"), or reading past the end of a list.
• The program stops at that line with a message.

Logic errors

• The program runs to the end but the result is wrong.
• Example: using - where you meant +, or stopping a loop too early.
• These are the hardest to find — the computer did exactly what you wrote.

Read the traceback

• When a program crashes, Python prints a traceback.
• Read the last line first: it names the error and what went wrong.
• The line number tells you where to look.

Traceback (most recent call last):
  File "<program>", line 2, in <module>
    print(10 / 0)
ZeroDivisionError: division by zero

Test with chosen data

• Do not trust code until you have tested it.
• Use normal data, boundary data (like 0), extreme data, and erroneous data that the program should reject.
• Check each answer against what you worked out by hand.

def square(n):
    return n * n

print(square(3))    # normal -> 9
print(square(0))    # boundary -> 0
print(square(-2))   # also normal: a negative number is valid input -> 4

Common mistakes

• A syntax error stops the program before it runs, so fix those first.
• Catch a specific error with except ValueError:, not a bare except:.
• Test the tricky cases (empty, zero, the biggest), not just the easy one.

Now you try

• Each task has a logic error to fix, or a crash to prevent.
• Press Check answer to test your fix against several cases.