🐍 Python Essentials

# Verify Python installation
python --version
# Output: Python 3.11.5

# Or on some systems:
python3 --version

# Verify pip installation
pip --version
# Output: pip 23.2.1 from ...

# Or on some systems:
pip3 --version

# My first Python program
# Save this as hello.py and run it

print("Hello, Python!")
print("Welcome to Code the Dream!")
print("You just wrote your first program 🎉")

# Let's do some math
print("Quick math:", 42 + 8)  # Should print 50

# Start the interactive shell
> python
Python 3.11.5 (main, Sep 11 2023, 13:54:46)
>>> 5 + 3
8
>>> "Hello" + " World"
'Hello World'
>>> len("Python")
6
>>> exit()  # Exit the shell

# Variables - storing data in labeled "boxes"
name = "Alice"        # str (string) - text data
age = 25              # int (integer) - whole numbers
height = 5.6          # float (decimal) - numbers with decimals
is_student = True    # bool (boolean) - True or False

# Print to console - see what's stored
print(f"Name: {name}")       # Output: Name: Alice
print(f"Age: {age}")         # Output: Age: 25
print(f"Is student: {is_student}") # Output: Is student: True

# User profile data (like what Instagram stores)
username = "@coder_alex"
followers = 1523
avg_likes_per_post = 87.5
verified_account = False

# Display profile summary
print(f"Profile: {username}")
print(f"Followers: {followers}")
print(f"Avg Likes: {avg_likes_per_post}")
print(f"Verified: {verified_account}")

# Change data (variables are mutable - they can change)
followers = 1524  # Someone new followed!
print(f"Updated Followers: {followers}")

# E-commerce shopping cart calculation
item_name = "Wireless Mouse"
item_price = 24.99
quantity = 2
tax_rate = 0.08  # 8% sales tax
free_shipping = True

# Calculate costs
subtotal = item_price * quantity
tax = subtotal * tax_rate
total = subtotal + tax

# Display receipt
print(f"=== RECEIPT ===")
print(f"Item: {item_name}")
print(f"Price: ${item_price} x {quantity}")
print(f"Subtotal: ${subtotal:.2f}")  # .2f = 2 decimal places
print(f"Tax (8%): ${tax:.2f}")
print(f"Total: ${total:.2f}")
print(f"Free Shipping: {free_shipping}")

# Output:
# === RECEIPT ===
# Item: Wireless Mouse
# Price: $24.99 x 2
# Subtotal: $49.98
# Tax (8%): $4.00
# Total: $53.98
# Free Shipping: True

# Arithmetic operators - Python as calculator
x = 10
y = 3

print(x + y)  # Addition: 13
print(x - y)  # Subtraction: 7
print(x * y)  # Multiplication: 30
print(x / y)  # Division: 3.333... (always returns float)
print(x // y) # Floor division: 3 (rounds down to nearest integer)
print(x % y)  # Modulo (remainder): 1 (10 ÷ 3 = 3 remainder 1)
print(x ** y) # Exponent: 1000 (10^3 = 10 × 10 × 10)

# Comparison operators - return True or False
age = 25
voting_age = 18

print(age > voting_age)   # True (25 is greater than 18)
print(age < voting_age)   # False (25 is not less than 18)
print(age >= 25)         # True (25 is greater than or equal to 25)
print(age <= 30)         # True (25 is less than or equal to 30)
print(age == 25)         # True (age IS equal to 25)
print(age != 30)         # True (age is NOT equal to 30)

# E-commerce discount logic
customer_age = 22
is_premium_member = True
purchase_amount = 150.00

# Business rules: 10% discount if premium OR spent over $100
qualifies_for_discount = is_premium_member or (purchase_amount > 100)

print(f"Customer Age: {customer_age}")
print(f"Premium Member: {is_premium_member}")
print(f"Purchase Amount: ${purchase_amount}")
print(f"Qualifies for Discount: {qualifies_for_discount}")  # True

# Calculate discount
if qualifies_for_discount:
    discount = purchase_amount * 0.10
    final_price = purchase_amount - discount
    print(f"Discount Applied: ${discount:.2f}")
    print(f"Final Price: ${final_price:.2f}")

age = 18

if age >= 18:
    print("You can vote!")
elif age >= 16:
    print("You can drive!")
else:
    print("You're still young!")

# Age verification with multiple thresholds
age = 20

if age >= 21:
    print("Full access: vote, drive, and purchase alcohol.")
elif age >= 18:
    print("Access: vote and drive.")
elif age >= 16:
    print("Access: learner’s permit to drive.")
else:
    print("Access: supervised activities only.")

# Grade calculator: A/B/C/D/F
score = 87

if score >= 90:
    letter = 'A'
elif score >= 80:
    letter = 'B'
elif score >= 70:
    letter = 'C'
elif score >= 60:
    letter = 'D'
else:
    letter = 'F'

print(f"Letter grade: {letter}")

# Tiered discounts: membership + purchase amount
is_premium = True
amount = 180.00

if is_premium and amount >= 150:
    discount = 0.20
elif is_premium:
    discount = 0.15
elif amount >= 100:
    discount = 0.10
else:
    discount = 0.00

final = amount * (1 - discount)
print(f"Discount: {discount*100:.0f}% | Total: ${final:.2f}")

# for loop - iterate over a sequence
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# range() - generate numbers
for i in range(5):  # 0, 1, 2, 3, 4
    print(i)

# while loop - repeat until condition is False
count = 0
while count < 5:
    print(count)
    count += 1

# break - exit loop early
for i in range(10):
    if i == 5:
        break  # Stop at 5
    print(i)

# continue - skip to next iteration
for i in range(5):
    if i == 2:
        continue  # Skip 2
    print(i)  # Prints 0, 1, 3, 4

# FizzBuzz - a classic interview question
for num in range(1, 21):
    if num % 3 == 0 and num % 5 == 0:
        print("FizzBuzz")
    elif num % 3 == 0:
        print("Fizz")
    elif num % 5 == 0:
        print("Buzz")
    else:
        print(num)

# Define a function
def greet(name):
    return f"Hello, {name}!"

# Call the function
message = greet("Alice")
print(message)  # Hello, Alice!

# Function with multiple parameters
def add(a, b):
    return a + b

result = add(5, 3)
print(result)  # 8

# Default parameters
def greet_with_time(name, time="day"):
    return f"Good {time}, {name}!"

print(greet_with_time("Bob"))            # Good day, Bob!
print(greet_with_time("Bob", "morning"))  # Good morning, Bob!

# Progressive: calculator functions
def add(a, b):
    return a + b

def subtract(a, b):
    return a - b

def calc_tax(amount, rate=0.08):
    return amount * rate

total = add(50, 20)
tax = calc_tax(total)
print(f"Total: ${total:.2f} | Tax: ${tax:.2f}")

# Data validation: return vs print
def is_valid_email(email):
    return '@' in email and '.' in email

result = is_valid_email('alice@example.com')
if result:
    print("Email looks valid.")
else:
    print("Please check the address.")

# Local vs Global Variables
message = "I'm global"  # Global - accessible everywhere

def greet():
    message = "I'm local"  # Local - only inside this function
    print(message)

greet()         # Output: I'm local
print(message)  # Output: I'm global

# Functions can return multiple values
def get_stats(purchases):
    total = sum(purchases)
    count = len(purchases)
    return total, count

purchases = [29.99, 49.99, 9.99]
total, count = get_stats(purchases)
print(f"Total: ${total:.2f}, Items: {count}")

# Handle errors gracefully
try:
    number = int(input("Enter a number: "))
    result = 100 / number
    print(f"Result: {result}")
except ValueError:
    print("That's not a valid number!")
except ZeroDivisionError:
    print("Can't divide by zero!")
except Exception as e:
    print(f"An error occurred: {e}")

def calculate_average(numbers):
    print(f"DEBUG: numbers = {numbers}")  # See what we're working with
    
    total = sum(numbers)
    print(f"DEBUG: total = {total}")       # Check intermediate result
    
    count = len(numbers)
    print(f"DEBUG: count = {count}")       # Verify count
    
    average = total / count
    return average

result = calculate_average([5, 10, 15])
print(f"Average: {result}")

# ═══ VARIABLES ═══
name = "Alice"
age = 25
height = 5.6
is_student = True

# ═══ OPERATORS ═══
x + y    # Addition
x - y    # Subtraction
x * y    # Multiplication
x / y    # Division
x // y   # Floor division
x % y    # Modulo (remainder)
x ** y   # Exponent

# ═══ CONTROL FLOW ═══
if condition:
    # do something
elif other_condition:
    # do something else
else:
    # do default

for item in sequence:
    # repeat for each item

while condition:
    # repeat while True

# ═══ FUNCTIONS ═══
def function_name(param1, param2="default"):
    # do something
    return result

# ═══ ERROR HANDLING ═══
try:
    # risky code
except ErrorType:
    # handle error