GCSE Python Docs

# Assigning variables
name = "Alice"       # String
age = 16             # Integer
height = 1.75        # Float
is_student = True    # Boolean

# Constants (Convention is UPPER_CASE)
PI = 3.14159
MAX_SCORE = 100

# Outputting to the console
print("Name:", name)
print(f"Age: {age}") 
print(f"Area: {PI * 5 * 5}") # Calculation inside f-string

# Checking types
print(type(name))   # 
print(type(age))    # 

# This is a single-line comment
x = 5 # Comments can go at the end of a line

'''
This is a multi-line comment.
It can span multiple lines.
Useful for explaining complex logic.
'''
print("Comments are invisible to the computer!")

# String to Integer
num_str = "50"
num_int = int(num_str) 

# Integer to String
score = 100
message = "Score: " + str(score)

# Input is always string, wrap in int() for maths
# Try running this and entering a number!
try:
    age = int(input("Enter your age: "))
    next_year = age + 1
    print(f"Next year you will be {next_year}")
except ValueError:
    print("Please enter a valid whole number")

text = "Computer Science"

# Length
print(len(text))        # 16

# Case conversion
print(text.upper())     # "COMPUTER SCIENCE"
print(text.lower())     # "computer science"

# Indexing (Starts at 0)
print(text[0])          # "C"
print(text[-1])         # "e" (last character)

# Slicing [start:stop]
print(text[0:8])        # "Computer" (stops before index 8)
print(text[9:])         # "Science" (to the end)

# Advanced Slicing [start:stop:step]
print(text[::-1])       # "ecneicS retupmoC" (Reverse)

# Replacement
new_text = text.replace("Science", "Programming")
print(new_text)         # "Computer Programming"

# Splitting to list
sentence = "Coding is fun"
words = sentence.split(" ") 
print(words)            # ['Coding', 'is', 'fun']

# Concatenation (Joining)
first = "Hello"
second = "World"
combined = first + " " + second # "Hello World"

# Example 1: Simple Grades
score = 75

if score >= 80:
    print("Grade: A")
elif score >= 60:
    print("Grade: B")
elif score >= 40:
    print("Grade: C")
else:
    print("Grade: U")
    
# Example 2: Nested Selection and Logic
age = 15
has_ticket = True

if has_ticket:
    if age >= 18:
        print("Allowed entry (Adult)")
    elif age >= 12:
        print("Allowed entry (Teen)")
    else:
        print("Too young for this movie")
else:
    print("Please buy a ticket first")

# Logical Operators: and, or, not
day = "Saturday"
sunny = True

if (day == "Saturday" or day == "Sunday") and sunny:
    print("Let's go to the beach!")

# Basic password check
password = ""
while password != "secret":
    password = input("Enter password: ")
    if password != "secret":
        print("Wrong password, try again.")
print("Access Granted")

# Using break to exit early
print("Type 'exit' to stop")
while True:
    user_input = input("> ")
    if user_input == "exit":
        print("Goodbye!")
        break
    print(f"You said: {user_input}")

fruits = ["Apple", "Banana", "Cherry"]

# Accessing items
print(fruits[0])      # Apple
print(len(fruits))    # Length is 3

# Modifying items
fruits[1] = "Blueberry"

# Adding items
fruits.append("Date")       # Adds to end
fruits.insert(0, "Apricot") # Adds to start

# Removing items
fruits.pop()          # Removes last item
fruits.remove("Apple") # Removes specific item

# Numerical Lists & Functions
numbers = [5, 2, 9, 1, 8]
print(max(numbers))   # 9
print(min(numbers))   # 1
print(sum(numbers))   # 25

numbers.sort()        # Sorts list in-place
print(numbers)        # [1, 2, 5, 8, 9]

# Looping through a list
for fruit in fruits:
    print(fruit)

# Creating a dictionary (record)
student = {
    "name": "Alex",
    "age": 16,
    "grade": "A",
    "is_enrolled": True
}

# Accessing data
print(student["name"])  # Alex

# Modifying data
student["age"] = 17

# Adding new data
student["email"] = "[email protected]"

# Looping through keys and values
for key, value in student.items():
    print(f"{key}: {value}")

# A 3x3 grid (like a Tic-Tac-Toe board)
grid = [
    ["X", "O", "X"],
    ["O", "X", "O"],
    ["O", "X", "O"]
]

print(grid[0][0])  # Top-left
print(grid[1][1])  # Center

# Changing a value
grid[2][2] = "X"

# Nested loop to print grid nicely
for row in grid:
    for item in row:
        print(item, end=" ")
    print() # New line at end of row

# Procedure (performs action, no return)
def greet(name, time_of_day="Day"): # Default parameter
    print(f"Good {time_of_day}, {name}!")

# Function (calculates and returns value)
def check_grade(score):
    if score >= 50:
        return "Pass"
    else:
        return "Fail"

# Main Program
greet("Sam", "Morning") 
greet("Alex") # Uses default "Day"

my_score = 65
result = check_grade(my_score)
print(f"Student got: {result}")

score = 0 # Global variable

def increase_score():
    global score # We must tell Python we want to use the global 'score'
    score = score + 1
    print(f"Score inside function: {score}")

increase_score()
print(f"Score outside function: {score}")

def local_demo():
    my_secret = "Hidden" # Local variable
    print(my_secret)

local_demo()
# print(my_secret) # This would cause an Error!

valid_input = False

while not valid_input:
    try:
        # This code might crash if user types text
        number = int(input("Enter a number to divide 100 by: "))
        result = 100 / number
        print(f"Result: {result}")
        valid_input = True # Only runs if lines above succeed
        
    except ValueError:
        print("Error: You must enter a whole number.")
        
    except ZeroDivisionError:
        print("Error: You cannot divide by zero.")
        
    except Exception as e:
        print(f"Unknown error occurred: {e}")

# Writing to a file ('w' overwrites, 'a' appends)
with open("data.txt", "w") as file:
    file.write("Hello World\n")
    file.write("Python is fun")

# Reading from a file ('r' read mode)
try:
    with open("data.txt", "r") as file:
        content = file.read() 
        print(content)
except FileNotFoundError:
    print("File does not exist yet.")

# Reading line by line into a list
with open("data.txt", "r") as file:
    lines = file.readlines()
    print(f"There are {len(lines)} lines in the file.")

import random
import time
import math

# --- Random ---
roll = random.randint(1, 10)
print(f"You rolled a {roll}")
print(f"Random choice: {random.choice(['Red', 'Blue'])}")

# --- Time ---
print("Wait for it...")
time.sleep(1) # Pauses program for 1 second
print("Done!")

# --- Math ---
print(f"Pi is roughly {math.pi}")
print(f"Square root of 16 is {math.sqrt(16)}")
print(f"Floor (round down): {math.floor(4.9)}")
print(f"Ceil (round up): {math.ceil(4.1)}")