B2.1 Programming Fundamentals

Variables

A variable is a named location in memory that holds data. In Python you create one just by assigning a value to it. You do not have to state the type, Python works it out.

my_number = 5
name = "Ivan"

You can add an optional type hint to make the intended type clear. Python does not enforce it, it just makes code easier to read.

my_number: int = 5

Declaring your main variables near the top of a program makes it easier to read and debug.

Data Types

The data type decides what can be stored in a variable and what you can do with it.

Global and Local Variables

count = 0            # global

def add_one():
    global count     # lets the function change the global
    count = count + 1

def greet():
    message = "hi"   # local, only exists inside greet()
    print(message)

add_one()
greet()
print(count)         # 1
print(message)       # NameError: message is not defined

Local variables are safer because nothing outside the function can accidentally change them. Use globals sparingly.

Why Data Types Matter

Practice Exercises

score = 10

def update():
    score = 50
    print(score)

update()
print(score)

Strings and Indexes

A string is a sequence of characters. Every character has an index, starting at 0. Negative indexes count from the end.

String Operations

All examples use s = "Old Town".

Most programs take user input as strings, so being comfortable with string handling makes your programs much more useful.

Practice Exercises

surname = input("Enter your surname: ")
password = surname[:3].lower() + "#" + str(len(surname)) + "!"
print("Your starting password is", password)

# Input: Kruchinin
# Output: Your starting password is kru#9!

What is Exception Handling?

An exception is unexpected behaviour while a program runs. If it is not handled, the program crashes. Exception handling lets the program deal with the problem and carry on.

Three Points of Failure

try, except, finally

Risky code goes in the try block. If it fails, the except block runs instead of the program crashing.

pets = ["Ben", "Leo", "Zeppo"]
try:
    print(pets[5])
except IndexError:
    print("An index out of range error has occurred")

# Output: An index out of range error has occurred

A finally block always runs, whether there was an error or not. Useful for cleanup like closing files.

try:
    print(pets[5])
except IndexError:
    print("An error has occurred")
finally:
    print("The program has ended safely")

# Output:
# An error has occurred
# The program has ended safely

Input Validation Example

This keeps asking until the user enters a valid age. It handles both wrong type and wrong range.

while True:
    try:
        age = int(input("Enter your age: "))
        if 0 <= age <= 120:
            break
        print("Age must be between 0 and 120")
    except ValueError:
        print("Please enter a whole number")

print("Age saved:", age)

Practice Exercises

try:
    x = int("abc")
    print("Converted")
except ValueError:
    print("Not a number")
finally:
    print("Done")

Not a number
Done

try:
    a = float(input("First number: "))
    b = float(input("Second number: "))
    print("Result:", a / b)
except ValueError:
    print("Please enter numbers only")
except ZeroDivisionError:
    print("You cannot divide by zero")

What is Debugging?

Using tools and techniques to find and fix errors in your code. Always know what inputs your code takes and what output it should give. If the results are wrong, use these techniques to track down why. You will use all of them in your IA.

1. Trace Tables

Track the value of every variable at each step to check it changes the way you expect. Example: counting warm days (over 20°C).

temperature = [23.7, 19.9, 23.8, 18.8, 12.5, 24.0]
warm_days = 0
for temp in temperature:
    if temp > 20:
        warm_days = warm_days + 1
print("The number of warm days this month:", warm_days)

2. Print Statements

Add print lines to show which loop, iteration or branch the program is in and what the variables hold at that moment. This shows whether the program runs the way you expect. Example: a selection sort with debug prints.

to_sort = [4, 56, 23, 2938, 29, 485, 2]

for i in range(len(to_sort) - 1):
    min_index = i
    print("Outer loop, i =", i)
    for j in range(i + 1, len(to_sort)):
        print("  Inner loop, j =", j)
        if to_sort[j] < to_sort[min_index]:
            min_index = j
            print("    New smallest at index", min_index)
    to_sort[i], to_sort[min_index] = to_sort[min_index], to_sort[i]

print(to_sort)   # [2, 4, 23, 29, 56, 485, 2938]

The output is a running log of exactly what the code is doing. Remove the debug prints once the bug is fixed.

3. Breakpoints

A breakpoint pauses the program at a chosen line so you can inspect every variable at that moment. In most IDEs (VS Code, PyCharm, Thonny) you click in the margin next to a line number to set one. You can also add breakpoint() directly in Python code.

4. Step by Step Execution

The IDE runs the code one line at a time. It highlights the current line and shows a live table of all variable values.

Practice Exercises

nums = [3, 8, 1, 6]
total = 0
for n in nums:
    if n > 2:
        total = total + n
print(total)

marks = [6, 7, 5, 4]
total = 0
for m in marks:
    total = total + m
    average = total / len(marks)
    print(average)

marks = [6, 7, 5, 4]
total = 0
for m in marks:
    total = total + m
average = total / len(marks)
print(average)   # 5.5