4.2 · Types of Errors

Goal: classify any program error as syntax, logic, or run-time and explain its symptoms.

The three types

Type	Detected when	Symptom	Example
Syntax	Compile / interpret time	Program won't run at all	print(x missing closing )
Logic	At run time, gives wrong output	Program runs but result is wrong	Using < when <= was intended
Run-time	During execution	Program crashes mid-way	Division by zero, file not found, index out of range

Syntax errors

The interpreter (Python) refuses to run the program until you fix the grammar.

# SyntaxError: '(' was never closed
print("hi"

# IndentationError: unexpected indent
if x > 0:
        print(x)
    print(x)        # mis-aligned!

Fix: read the line number, find the typo.

Logic errors

Most insidious. The program runs and produces output — but the output is wrong.

# Wants: "Pass" if score >= 50, else "Fail"
if score > 50:           # BUG: should be >=
    print("Pass")
else:
    print("Fail")

Score = 50 → incorrectly prints "Fail".

Logic errors are detected by testing, not by Python.

Run-time errors (exceptions)

The program runs, but an unexpected condition crashes it.

Error	Cause
ZeroDivisionError	Divide by 0
ValueError	int("abc")
IndexError	lst[10] on a 3-element list
KeyError	Missing dict key
FileNotFoundError	Opening a missing file
TypeError	"5" + 3

Handle them with try / except:

try:
    n = int(input("Number: "))
    print(100 / n)
except ZeroDivisionError:
    print("Can't divide by zero")
except ValueError:
    print("That's not a number")

Mapping errors to their detection

Error type	Best caught by
Syntax	Linter / IDE syntax highlighting
Logic	Unit tests + trace tables + manual review
Run-time	Defensive code (try/except), good test data

Common student mistakes

• Confusing syntax errors with logic errors.
• Adding excessive try/except to hide bugs instead of fixing them.
• Treating Python print debugging as undignified — it's a perfectly fine technique.

Exam-style question

Q (4 marks): Identify the type of error in each snippet:

(a) print("Hello World" (b) total = 0; for i in range(1,11): total += i; print("Sum = ", total + 1) (off-by-one) (c) mark = int("87a") (d) result = 10 / 0

Sample answer:

(a) Syntax — unmatched parenthesis. (b) Logic — + 1 shouldn't be there; produces wrong sum. (c) Run-time (ValueError) — string contains non-numeric character. (d) Run-time (ZeroDivisionError).

Key takeaways

• 3 types: syntax, logic, run-time.
• Syntax: caught early. Logic: hardest. Run-time: handle gracefully.

➡️ Next: 4.3 Debugging Techniques