1   Errors Are a Normal Part of Programming

Every programmer — from complete beginner to senior engineer with twenty years of experience — writes code that produces errors. The goal is not to write perfect code on the first attempt. The goal is to read the error, understand it, and fix it efficiently.

There are three broad categories of error in Python. Understanding which category you are dealing with changes how you approach the fix:

2   Anatomy of an Error Message

When Python encounters a runtime error, it prints a traceback. A traceback is a report showing the chain of function calls that led to the crash, followed by the error type and a description. Here is a real example:

Traceback (most recent call last):
  File "mission.py", line 12, in <module>
    result = calculate_fuel(speed, distance)
  File "mission.py", line 7, in calculate_fuel
    return distance / speed
ZeroDivisionError: division by zero

This looks intimidating at first. Let's dissect every part:

3   Where to Find Error Messages in VSCode

In VSCode, when you run a Python file, output and errors both appear in the Terminal panel at the bottom of the screen. If the terminal is not visible, open it with Ctrl + ` (backtick) on Windows/Linux, or Cmd + ` on macOS.

Error messages appear in red text. Normal program output appears in white. A typical session looks like this:

PS C:\Users\student\projects> python mission.py

Traceback (most recent call last):
  File "mission.py", line 12, in <module>
    result = calculate_fuel(speed, distance)
  File "mission.py", line 7, in calculate_fuel
    return distance / speed
ZeroDivisionError: division by zero

3.1   Clicking the Line Number in VSCode

VSCode makes one part of the traceback interactive: the filename and line number (e.g. mission.py, line 7) appears as a clickable link in the terminal. Clicking it jumps your cursor directly to that line in the editor. This is one of VSCode's most useful features — use it every time.

3.2   The Red Underline in the Editor

For syntax errors, VSCode often catches the problem before you even run the file — a red squiggly underline appears under the offending code. Hovering over it shows a tooltip with the error description. This is the fastest possible feedback loop: fix the syntax before you run, not after.

4   The Common Error Types

Python has a specific name for every category of runtime error. Learning these names means you can recognise the problem the moment you see the last line of any traceback. Here are the eight you will encounter most often in Grade 9.

4.1   SyntaxError — "I can't read this"

A SyntaxError means Python cannot parse your code at all. The program never starts. Python found something that does not follow the rules of the language — a missing colon, an unclosed bracket, a misplaced keyword.

# Common causes of SyntaxError:

if score > 50          # missing colon
    print("pass")

print("hello"          # unclosed parenthesis

def calculate(a, b)    # missing colon after def
    return a + b

What you see in the terminal:

  File "ship.py", line 1
    if score > 50
                 ^
SyntaxError: expected ':'

The caret symbol ^ points to the position where Python got confused. Note that Python's pointer sometimes sits one character after the actual mistake — look at the character just before the caret too.

4.2   IndentationError — "This line is in the wrong place"

Python uses indentation to define structure. An IndentationError means a line's indentation does not match what Python expects — either too many spaces, too few, or a mix of tabs and spaces.

def greet(name):
print("Hello,", name)    # should be indented

  File "ship.py", line 2
    print("Hello,", name)
    ^
IndentationError: expected an indented block after function definition on line 1

4.3   NameError — "I've never heard of that"

A NameError means Python encountered a name — a variable or function — that has not been defined yet at that point in the program.

print(ship_name)         # ship_name was never assigned

Traceback (most recent call last):
  File "ship.py", line 1, in <module>
    print(ship_name)
          ^^^^^^^^^
NameError: name 'ship_name' is not defined

Common causes:

• Typo in the variable name: shiip_name vs ship_name.
• Using a variable before assigning it.
• Using a local variable outside the function it was created in (scope — see Article 6).
• Calling a function before defining it.
• Forgetting to import a module: math.sqrt(4) without import math.

4.4   TypeError — "You can't do that with those types"

A TypeError means you tried to perform an operation on a value of the wrong type — adding a string to an integer, calling something that is not a function, or passing the wrong number of arguments.

age = input("Enter your age: ")
next_year = age + 1              # age is a string — can't add int to str

Traceback (most recent call last):
  File "ship.py", line 2, in <module>
    next_year = age + 1
                ~~~^~~
TypeError: can only concatenate str (not "int") to str

Common causes:

• Forgetting to convert input() with int() or float().
• Using + to concatenate a string and a number without converting.
• Calling a non-function: result = 42().
• Passing the wrong number of arguments to a function.

The error message usually tells you exactly which types are involved — read it carefully:

# "can only concatenate str (not 'int') to str"
# Translation: the left side is a string, the right side is an int — fix the types first.

4.5   ValueError — "The type is right, but the value doesn't make sense"

A ValueError means the type of your data is correct, but the actual content is not valid for the operation you are trying to perform.

age = int("hello")          # the type is str — that's fine for int()
                             # but "hello" cannot be converted to an integer

Traceback (most recent call last):
  File "ship.py", line 1, in <module>
    age = int("hello")
          ^^^^^^^^^^^^
ValueError: invalid literal for int() with base 10: 'hello'

Common causes:

• Calling int() or float() on a string that is not a number.
• Calling .remove() on a list when the item is not present.
• Calling .index() on a list or string when the item is not present.

4.6   IndexError — "That position doesn't exist"

An IndexError means you tried to access a list or string at an index that is out of range — past the end (or before the beginning with a negative index that is too large).

crew = ["Alice", "Bob", "Carol"]
print(crew[5])               # only indices 0, 1, 2 exist

Traceback (most recent call last):
  File "ship.py", line 2, in <module>
    print(crew[5])
          ~~~~^^^
IndexError: list index out of range

Common causes:

• Using a hardcoded index that is too large for the actual list.
• Off-by-one error — using range(len(list)) correctly but then accidentally accessing list[len(list)].
• An empty list: trying to access my_list[0] when the list is [].
• A loop that runs one iteration too many.

4.7   KeyError — "That key doesn't exist in this dictionary"

A KeyError is the dictionary equivalent of an IndexError. You tried to access a key that is not in the dictionary.

ship = {"name": "Persephone", "fuel": 87.5}
print(ship["shields"])       # "shields" key does not exist

Traceback (most recent call last):
  File "ship.py", line 2, in <module>
    print(ship["shields"])
          ~~~~^^^^^^^^^^^
KeyError: 'shields'

Common causes:

• Typo in the key name: "sheild" instead of "shield".
• Assuming a key exists when it was never added.
• Case mismatch: "Name" vs "name".

The fix: use in to check before accessing, or use .get() (see Article 9).

4.8   ZeroDivisionError — "You can't divide by zero"

Python raises this when the right-hand side of a division or modulo operation is zero. This is mathematically undefined, so Python refuses to proceed.

speed = 0
time = 100 / speed           # division by zero

Traceback (most recent call last):
  File "ship.py", line 2, in <module>
    time = 100 / speed
           ~~~~^~~~~~~
ZeroDivisionError: division by zero

Common causes:

• A variable that should hold a non-zero number but was initialised to 0 and never updated.
• User input that is 0 when the program did not guard against it.
• Calculating an average on an empty list: sum(items) / len(items) when items is [].

5   Reading a Traceback Step by Step

Let's practise the three-step reading strategy on a realistic traceback. Here is a program and its error output:

5.1   The Program

# fleet_report.py

def get_average_fuel(fleet):
    total = 0
    for ship in fleet:
        total += ship["feul"]       # typo: "feul" instead of "fuel"
    return total / len(fleet)

fleet = [
    {"name": "Persephone", "fuel": 87.5},
    {"name": "Icarus IV",  "fuel": 43.0},
]

average = get_average_fuel(fleet)
print("Average fuel:", average)

5.2   The Error Output

Traceback (most recent call last):
  File "fleet_report.py", line 14, in <module>
    average = get_average_fuel(fleet)
  File "fleet_report.py", line 5, in get_average_fuel
    total += ship["feul"]
             ~~~~^^^^^^^^
KeyError: 'feul'

5.3   Applying the Three Steps

The fix: change ship["feul"] to ship["fuel"] on line 5.

6   A Quick-Reference Catalogue of Error Messages

Here are the most common error messages you will see in Grade 9, with plain-English translations and the most likely fix for each.

7   A Debugging Workflow

Having a consistent process for fixing bugs makes the whole experience less stressful. Here is a workflow that works for beginners and professionals alike:

8   Worked Examples — Read, Diagnose, Fix

Here are four complete worked examples. For each one: read the code, read the error, apply the three-step strategy, and find the fix before reading the explanation.

Example 1

# Code
crew_count = 12
message = "Crew aboard: " + crew_count
print(message)

# Error
TypeError: can only concatenate str (not "int") to str

Diagnosis: crew_count is an integer. The + operator cannot concatenate a string and an integer.
Fix: message = "Crew aboard: " + str(crew_count)

Example 2

# Code
planets = ["Mercury", "Venus", "Earth", "Mars"]
for i in range(1, 5):
    print(planets[i])

# Error
IndexError: list index out of range

Diagnosis: The list has 4 items at indices 0–3. The loop runs i = 1, 2, 3, 4. When i = 4, planets[4] does not exist.
Fix: Change range(1, 5) to range(1, 4) — or better, use range(len(planets)), or simply for planet in planets.

Example 3

# Code
def warp_speed(distance, time)
    return distance / time

print(warp_speed(100, 5))

# Error
  File "ship.py", line 2
    def warp_speed(distance, time)
                                  ^
SyntaxError: expected ':'

Diagnosis: The def line is missing its colon.
Fix: def warp_speed(distance, time):

Example 4

# Code
def calculate_average(scores):
    total = sum(scores)
    average = total / len(scores)
    return average

result = calculate_average([])
print("Average:", result)

# Error
ZeroDivisionError: division by zero

Diagnosis: An empty list was passed in. len([]) is 0, causing division by zero.
Fix: Guard against an empty list before dividing:

def calculate_average(scores):
    if len(scores) == 0:
        return 0
    return sum(scores) / len(scores)

9   Final Check: The Big Picture

10   Quick-Reference: Error Types at a Glance