# Python infers the type automatically
name = "Alice"          # str (string)
age = 25                 # int (integer)
height = 5.7             # float (decimal)
is_student = True        # bool (boolean)

print(name, age, height, is_student)

x = "42"
print(type(x))         # <class 'str'>

x = int(x)              # Convert string to int
print(type(x))         # <class 'int'>
print(x + 8)           # 50

# Other conversions
float("3.14")           # 3.14
str(100)                # "100"
bool(0)                 # False (0 is falsy)
bool(1)                 # True  (non-zero is truthy)

# Assign multiple variables at once
a, b, c = 1, 2, 3

# Swap values (no temp variable needed!)
a, b = b, a
print(a, b)  # 2 1

single = 'Hello'
double = "World"
multi = """This is a
multi-line string"""

# String concatenation
full = single + " " + double
print(full)  # Hello World

name = "Alice"
age = 25

# f-string — note the f before the quote
message = f"My name is {name} and I am {age} years old."
print(message)

# You can put expressions inside {}
print(f"Next year I'll be {age + 1}")
print(f"Name uppercase: {name.upper()}")

s = "Python"

print(s[0])      # P   (first character)
print(s[-1])     # n   (last character)
print(s[0:3])    # Pyt (index 0,1,2 — end is exclusive)
print(s[2:])     # thon (from index 2 to end)
print(s[::2])     # Pto (every 2nd character)
print(s[::-1])    # nohtyP (reversed!)

print(5 == 5)     # True  (equal)
print(5 != 3)     # True  (not equal)
print(5 > 3)      # True  (greater than)
print(5 <= 5)    # True  (less than or equal)

x = 15
print(x > 10 and x < 20)   # True  (both true)
print(x > 20 or x < 5)    # False (neither true)
print(not (x > 20))         # True  (negation)

# input() always returns a string!
name = input("What's your name? ")
print(f"Hello, {name}!")

# Convert to number for math
age = int(input("Enter your age: "))
print(f"You'll be {age + 1} next year!")

score = 85

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
else:
    print("Grade: F")

age = 20
status = "adult" if age >= 18 else "minor"
print(status)  # adult

name = ""
if name:
    print(f"Hello, {name}")
else:
    print("Name is empty!")  # This runs

# Loop through a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

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

for i in range(2, 10, 2):  # 2, 4, 6, 8 (start, stop, step)
    print(i)

# enumerate() — get index AND value
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

count = 0
while count < 5:
    print(f"Count: {count}")
    count += 1  # Don't forget this — or infinite loop!

# break — exit loop immediately
for i in range(10):
    if i == 5:
        break
    print(i)  # prints 0, 1, 2, 3, 4

# continue — skip this iteration
for i in range(5):
    if i == 2:
        continue
    print(i)  # prints 0, 1, 3, 4 (skips 2)

# Traditional way
squares = []
for x in range(10):
    squares.append(x ** 2)

# List comprehension — same result, one line!
squares = [x ** 2 for x in range(10)]

# With a filter
evens = [x for x in range(20) if x % 2 == 0]
print(evens)  # [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

# Creating lists
fruits = ["apple", "banana", "cherry"]
numbers = [1, 2, 3, 4, 5]
mixed = [1, "hello", True, 3.14]

# Accessing elements (same as strings)
print(fruits[0])     # apple
print(fruits[-1])    # cherry
print(fruits[1:3])   # ['banana', 'cherry']

fruits = ["apple", "banana"]

fruits.append("cherry")        # Add to end
fruits.insert(1, "blueberry")  # Add at index 1
fruits.remove("banana")        # Remove by value
popped = fruits.pop()           # Remove & return last
fruits.sort()                   # Sort in place
fruits.reverse()                # Reverse in place
print(len(fruits))              # Length
print("apple" in fruits)       # True (membership test)

point = (10, 20)
rgb = (255, 128, 0)

print(point[0])  # 10

# Tuple unpacking
x, y = point
print(x, y)      # 10 20

# point[0] = 5    # ❌ TypeError! Tuples are immutable

person = {
    "name": "Alice",
    "age": 25,
    "city": "Mumbai"
}

# Access values
print(person["name"])         # Alice
print(person.get("age"))      # 25 (safer — returns None if missing)

# Modify
person["age"] = 26            # Update
person["email"] = "a@b.com"  # Add new key
del person["city"]             # Delete key

# Loop through a dict
for key, value in person.items():
    print(f"{key}: {value}")

colors = {"red", "blue", "green", "red"}
print(colors)  # {'red', 'blue', 'green'} — no duplicates!

# Remove duplicates from a list
nums = [1, 2, 2, 3, 3, 3]
unique = list(set(nums))  # [1, 2, 3]

# Set operations
a = {1, 2, 3}
b = {3, 4, 5}
print(a | b)   # {1, 2, 3, 4, 5}  — union
print(a & b)   # {3}              — intersection
print(a - b)   # {1, 2}           — difference

def greet(name):
    """Greet a person by name."""
    return f"Hello, {name}!"

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

# Default parameters
def power(base, exponent=2):
    return base ** exponent

print(power(3))       # 9   (uses default exponent=2)
print(power(3, 3))    # 27

# Keyword arguments
print(power(exponent=4, base=2))  # 16

# *args — variable number of arguments
def add_all(*numbers):
    return sum(numbers)

print(add_all(1, 2, 3, 4))  # 10

# **kwargs — variable keyword arguments
def build_profile(**info):
    return info

print(build_profile(name="Alice", age=25))
# {'name': 'Alice', 'age': 25}

square = lambda x: x ** 2
print(square(5))   # 25

# Useful with sorted(), map(), filter()
names = ["charlie", "alice", "bob"]
print(sorted(names, key=lambda n: len(n)))
# ['bob', 'alice', 'charlie']

# Import entire module
import math
print(math.sqrt(16))     # 4.0
print(math.pi)            # 3.14159...

# Import specific things
from random import randint, choice
print(randint(1, 100))   # Random int between 1-100

# Import with alias
import datetime as dt
today = dt.date.today()
print(today)              # 2026-04-08

# helpers.py — your custom module
def celsius_to_fahrenheit(c):
    return c * 9/5 + 32

def fahrenheit_to_celsius(f):
    return (f - 32) * 5/9

# main.py — use your module
from helpers import celsius_to_fahrenheit

print(celsius_to_fahrenheit(37))  # 98.6

# Install a package
pip install requests

# Then use it
import requests
response = requests.get("https://api.github.com")
print(response.status_code)  # 200

class Dog:
    # Constructor — runs when you create an instance
    def __init__(self, name, breed):
        self.name = name       # Instance attribute
        self.breed = breed
        self.energy = 100

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

    def play(self):
        self.energy -= 20
        return f"{self.name} plays! Energy: {self.energy}"

# Create instances (objects)
rex = Dog("Rex", "Labrador")
bella = Dog("Bella", "Poodle")

print(rex.bark())     # Rex says Woof!
print(bella.play())   # Bella plays! Energy: 80

class Animal:
    def __init__(self, name, species):
        self.name = name
        self.species = species

    def speak(self):
        return f"{self.name} makes a sound."

class Cat(Animal):    # Cat inherits from Animal
    def __init__(self, name, indoor=True):
        super().__init__(name, "Cat")  # Call parent's __init__
        self.indoor = indoor

    def speak(self):     # Override parent method
        return f"{self.name} says Meow!"

whiskers = Cat("Whiskers")
print(whiskers.speak())      # Whiskers says Meow!
print(whiskers.species)     # Cat (inherited)

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __repr__(self):      # String representation
        return f"Point({self.x}, {self.y})"

    def __add__(self, other):  # Enable + operator
        return Point(self.x + other.x, self.y + other.y)

    def __eq__(self, other):   # Enable == comparison
        return self.x == other.x and self.y == other.y

p1 = Point(1, 2)
p2 = Point(3, 4)
print(p1 + p2)  # Point(4, 6)

# Best practice: use 'with' — auto-closes the file
with open("data.txt", "r") as file:
    content = file.read()        # Read entire file
    print(content)

# Read line by line (memory efficient for large files)
with open("data.txt") as file:
    for line in file:
        print(line.strip())    # strip() removes newline

# Write (creates or overwrites file)
with open("output.txt", "w") as file:
    file.write("Hello, File!\n")
    file.write("Second line.\n")

# Append (add to end without overwriting)
with open("output.txt", "a") as file:
    file.write("Appended line.\n")

import json

# Write JSON
data = {"name": "Alice", "scores": [95, 87, 92]}
with open("data.json", "w") as f:
    json.dump(data, f, indent=2)

# Read JSON
with open("data.json") as f:
    loaded = json.load(f)
print(loaded["name"])  # Alice

try:
    num = int(input("Enter a number: "))
    result = 100 / num
    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"Unexpected error: {e}")
else:
    print("No errors occurred!")  # Runs only if no exception
finally:
    print("This ALWAYS runs.")    # Cleanup

def set_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative!")
    return age

try:
    set_age(-5)
except ValueError as e:
    print(e)  # Age cannot be negative!