Python Programming Fundamentals — Syntax, Data Structures, OOP & Core Library

# ✅ IDIOMATIC PYTHON: Clear, clean, whitespace-delimited
def calculate_discount(price: float, is_member: bool) -> float:
    # 4 spaces indent for function body
    if is_member:
        # 8 spaces indent for conditional block
        discount = price * 0.15
        return price - discount
    
    return price

# Comments start with '#'
# Multi-line docstrings use triple quotes:
"""
This is a module or function docstring explaining intent.
"""

username = "Alex"
score = 87.4567
rank = 3

# F-strings evaluate expressions inline with rich formatting:
print(f"Player: {username.upper()}")
print(f"Final Score: {score:.2f}")           # Formatted to 2 decimal places -> 87.46
print(f"Rank with padding: {rank:03d}")        # Padded with zeros -> 003
print(f"Calculation: 10 * 5 = {10 * 5}")      # Inline arithmetic -> 50

def handle_command(command: str | list):
    match command:
        case "quit":
            print("Exiting application...")
        case ["load", filename]:
            print(f"Loading file: {filename}")
        case ["set", key, value]:
            print(f"Setting config: {key} = {value}")
        case _:
            print(f"Unknown command: {command}")

# Type hints (PEP 484 / PEP 604) document contracts:
def build_profile(
    name: str,
    email: str,
    tier: str = "Standard",        # Default argument
    *interests: str,               # Variadic positional arguments (*args -> tuple)
    **extra_metadata: any          # Variadic keyword arguments (**kwargs -> dict)
) -> dict:
    return {
        "name": name,
        "email": email,
        "tier": tier,
        "interests": list(interests),
        "metadata": extra_metadata
    }

# Calling with mixed positional, args, and kwargs:
user = build_profile(
    "Samantha", 
    "sam@example.com", 
    "Premium", 
    "AI", "Security", "Python",    # Collected into *interests
    city="Seattle", verified=True   # Collected into **extra_metadata
)

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# List comprehension: [expression for item in iterable if condition]
even_squares = [n ** 2 for n in numbers if n % 2 == 0]
# -> [4, 16, 36, 64, 100]

# Dictionary comprehension: {key: value for item in iterable}
names = ["alice", "bob", "charlie"]
name_lengths = {name: len(name) for name in names}
# -> {'alice': 5, 'bob': 3, 'charlie': 7}

# Defining a custom exception inheriting from built-in Exception:
class NegativeValueError(Exception):
    """Raised when an unexpected negative number is supplied."""
    pass

def process_payment(amount: float) -> str:
    try:
        if amount < 0:
            raise NegativeValueError("Amount cannot be negative!")
        fee = 50.0 / amount
    except ZeroDivisionError as err:
        return f"Caught error: Amount cannot be zero ({err})"
    except NegativeValueError as err:
        return f"Validation error: {err}"
    except Exception as err:
        return f"Unexpected fallback error: {err}"
    else:
        # Runs ONLY if no exception was raised in the try block
        return f"Success! Fee: {fee:.2f}"
    finally:
        # Runs ALWAYS (for cleanup, releasing resources)
        print("Payment processing transaction logged.")

class InventoryItem:
    # Class attribute (shared by all instances)
    category = "General Goods"

    # Constructor method initializes instance attributes:
    def __init__(self, sku: str, name: str, price: float, stock: int = 0):
        self.sku = sku
        self.name = name
        self.price = price
        self.stock = stock

    # Instance method (must accept 'self' as first parameter)
    def purchase(self, quantity: int) -> float:
        if quantity > self.stock:
            raise ValueError(f"Insufficient stock for {self.name} (Available: {self.stock})")
        self.stock -= quantity
        return self.price * quantity

    # String representation for printing and debugging:
    def __str__(self) -> str:
        return f"{self.name} [SKU: {self.sku}] - ${self.price:.2f} ({self.stock} in stock)"

item = InventoryItem("SKU-101", "Mechanical Keyboard", 89.99, stock=15)
print(item)  # Calls __str__