Python Fundamentals

# Variables - no type declaration needed!
name = "Alice"           # String (text)
age = 25                 # Integer (whole number)
height = 5.6             # Float (decimal number)
is_student = True        # Boolean (True/False)

# Python is dynamically typed
x = 10          # x is an integer
x = "hello"     # now x is a string (no error!)

# Multiple assignment
a, b, c = 1, 2, 3
x = y = z = 0   # all get the same value

# Basic data types
text = "Hello World"        # str
number = 42                 # int
decimal = 3.14             # float
is_valid = True            # bool
nothing = None             # NoneType (similar to null)

# Creating strings
single = 'Hello'
double = "World"
multi_line = """This is
a multi-line
string"""

# String operations
greeting = "Hello" + " " + "World"  # Concatenation
repeated = "Ha" * 3                 # "HaHaHa"

# String methods
text = "  Python Programming  "
print(text.lower())         # "  python programming  "
print(text.upper())         # "  PYTHON PROGRAMMING  "
print(text.strip())         # "Python Programming"
print(text.replace("Python", "Java"))  # "  Java Programming  "

# String formatting (modern way)
name = "Alice"
age = 25
message = f"My name is {name} and I'm {age} years old"
# Output: "My name is Alice and I'm 25 years old"

# String slicing
word = "Python"
print(word[0])      # "P" (first character)
print(word[-1])     # "n" (last character)
print(word[0:3])    # "Pyt" (characters 0 to 2)
print(word[:3])     # "Pyt" (from start to 2)
print(word[3:])     # "hon" (from 3 to end)

# Creating lists
fruits = ["apple", "banana", "orange"]
numbers = [1, 2, 3, 4, 5]
mixed = [1, "hello", True, 3.14]  # Can mix types!

# Accessing items
print(fruits[0])     # "apple"
print(fruits[-1])    # "orange" (last item)

# List methods
fruits.append("grape")          # Add to end
fruits.insert(0, "mango")      # Insert at position
fruits.remove("banana")         # Remove specific item
last_fruit = fruits.pop()      # Remove and return last item

# List operations
combined = fruits + numbers     # Combine lists
print(len(fruits))             # Length of list
print("apple" in fruits)       # Check if item exists (True)

# List slicing
numbers = [0, 1, 2, 3, 4, 5]
print(numbers[1:4])   # [1, 2, 3]
print(numbers[::2])   # [0, 2, 4] (every 2nd item)

# List comprehension (elegant way to create lists)
squares = [x**2 for x in range(5)]
# [0, 1, 4, 9, 16]

even_numbers = [x for x in range(10) if x % 2 == 0]
# [0, 2, 4, 6, 8]

# Creating dictionaries
person = {
    "name": "Alice",
    "age": 25,
    "city": "New York",
    "is_student": True
}

# Accessing values
print(person["name"])           # "Alice"
print(person.get("age"))        # 25
print(person.get("country", "USA"))  # "USA" (default if key not found)

# Modifying dictionaries
person["age"] = 26              # Update existing
person["email"] = "alice@example.com"  # Add new key

# Dictionary methods
print(person.keys())            # All keys
print(person.values())          # All values
print(person.items())           # Key-value pairs

# Looping through dictionaries
for key, value in person.items():
    print(f"{key}: {value}")

# Nested dictionaries (real-world example)
users = {
    "user1": {"name": "Alice", "age": 25},
    "user2": {"name": "Bob", "age": 30}
}
print(users["user1"]["name"])  # "Alice"

# Basic function
def greet():
    print("Hello, World!")

greet()  # Call the function

# Function with parameters
def greet_person(name):
    print(f"Hello, {name}!")

greet_person("Alice")  # "Hello, Alice!"

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

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

# Default parameters
def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"

print(greet("Alice"))              # "Hello, Alice!"
print(greet("Bob", "Hi"))          # "Hi, Bob!"

# Multiple return values
def calculate(a, b):
    sum_val = a + b
    diff_val = a - b
    return sum_val, diff_val

total, difference = calculate(10, 5)
# total = 15, difference = 5

# Lambda functions (one-line functions)
square = lambda x: x**2
print(square(5))  # 25

# Real-world example
def calculate_discount(price, discount_percent=10):
    """Calculate final price after discount"""
    discount = price * (discount_percent / 100)
    final_price = price - discount
    return final_price

print(calculate_discount(100))      # 90.0
print(calculate_discount(100, 20))  # 80.0

# IF statements
age = 18

if age >= 18:
    print("You can vote!")
elif age >= 16:
    print("You can get a driver's license!")
else:
    print("You're still young!")

# FOR loops (iterate over sequences)
fruits = ["apple", "banana", "orange"]

for fruit in fruits:
    print(f"I like {fruit}")

# Loop with range
for i in range(5):
    print(i)  # Prints 0, 1, 2, 3, 4

# WHILE loops (continue until condition is False)
count = 0
while count < 5:
    print(count)
    count += 1

# Break and continue
for i in range(10):
    if i == 3:
        continue  # Skip this iteration
    if i == 7:
        break     # Exit the loop
    print(i)

# Real-world example: validate user input
while True:
    user_input = input("Enter a positive number: ")
    if user_input.isdigit() and int(user_input) > 0:
        number = int(user_input)
        break
    else:
        print("Invalid input. Try again.")

# Define a class (blueprint)
class Dog:
    # Constructor (runs when object is created)
    def __init__(self, name, age):
        self.name = name    # Instance variable
        self.age = age

    # Method (function inside a class)
    def bark(self):
        return f"{self.name} says Woof!"

    def get_age_in_dog_years(self):
        return self.age * 7

# Create objects (instances of the class)
my_dog = Dog("Buddy", 3)
your_dog = Dog("Max", 5)

print(my_dog.bark())                    # "Buddy says Woof!"
print(my_dog.get_age_in_dog_years())   # 21
print(your_dog.name)                    # "Max"

# Real-world example: Bank Account
class BankAccount:
    def __init__(self, owner, balance=0):
        self.owner = owner
        self.balance = balance

    def deposit(self, amount):
        self.balance += amount
        return f"Deposited ${amount}. New balance: ${self.balance}"

    def withdraw(self, amount):
        if amount > self.balance:
            return "Insufficient funds!"
        self.balance -= amount
        return f"Withdrew ${amount}. New balance: ${self.balance}"

    def get_balance(self):
        return f"{self.owner}'s balance: ${self.balance}"

# Using the class
account = BankAccount("Alice", 1000)
print(account.deposit(500))     # "Deposited $500. New balance: $1500"
print(account.withdraw(200))    # "Withdrew $200. New balance: $1300"
print(account.get_balance())    # "Alice's balance: $1300"

# Inheritance (creating classes based on other classes)
class SavingsAccount(BankAccount):
    def __init__(self, owner, balance=0, interest_rate=0.02):
        super().__init__(owner, balance)
        self.interest_rate = interest_rate

    def add_interest(self):
        interest = self.balance * self.interest_rate
        self.balance += interest
        return f"Added interest: ${interest}"

savings = SavingsAccount("Bob", 1000)
print(savings.add_interest())  # "Added interest: $20.0"

# Basic error handling
try:
    number = int(input("Enter a number: "))
    result = 10 / number
    print(f"Result: {result}")
except ValueError:
    print("That's not a valid number!")
except ZeroDivisionError:
    print("Cannot divide by zero!")
except Exception as e:
    print(f"An error occurred: {e}")
finally:
    print("This always runs, error or not")

# Real-world example: file reading
def read_file(filename):
    try:
        with open(filename, 'r') as file:
            content = file.read()
            return content
    except FileNotFoundError:
        return f"Error: {filename} not found"
    except PermissionError:
        return "Error: No permission to read file"

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

# Reading from a file
with open('data.txt', 'r') as file:
    content = file.read()
    print(content)

# Reading line by line
with open('data.txt', 'r') as file:
    for line in file:
        print(line.strip())

# Appending to a file
with open('data.txt', 'a') as file:
    file.write('\nNew line added!')

# Working with JSON (common for APIs)
import json

# Write JSON
data = {"name": "Alice", "age": 25, "city": "NYC"}
with open('data.json', 'w') as file:
    json.dump(data, file, indent=2)

# Read JSON
with open('data.json', 'r') as file:
    loaded_data = json.load(file)
    print(loaded_data["name"])  # "Alice"