Bài 19: Python Best Practices (Phần 2)

SOLID Principles

SOLID là 5 principles giúp code maintainable, scalable, và testable.

1. Single Responsibility Principle (SRP)

"A class should have only one reason to change."

Mỗi class chỉ làm một việc duy nhất.

❌ Violating SRP

class User:    """User class doing too many things."""        def __init__(self, username, email):        self.username = username        self.email = email        def save_to_database(self):        """Save user to database - DB responsibility"""        # Database logic        pass        def send_welcome_email(self):        """Send email - Email responsibility"""        # Email logic        pass        def generate_report(self):        """Generate report - Reporting responsibility"""        # Report logic        pass        def validate_email(self):        """Validate email - Validation responsibility"""        # Validation logic        pass

✅ Following SRP

class User:    """User model - only handles user data."""        def __init__(self, username: str, email: str):        self.username = username        self.email = email        def get_display_name(self) -> str:        """Get user display name."""        return self.username class UserRepository:    """Handles user database operations."""        def save(self, user: User):        """Save user to database."""        # Database logic        pass        def find_by_username(self, username: str) -> User:        """Find user by username."""        # Database query        pass class EmailService:    """Handles email operations."""        def send_welcome_email(self, user: User):        """Send welcome email to user."""        # Email logic        pass class UserValidator:    """Validates user data."""        def validate_email(self, email: str) -> bool:        """Validate email format."""        import re        pattern = r'^[\w\.-]+@[\w\.-]+\.\w+$'        return bool(re.match(pattern, email)) class ReportGenerator:    """Generates reports."""        def generate_user_report(self, user: User) -> str:        """Generate user report."""        return f"Report for {user.username}" # Sử dụnguser = User("john_doe", "john@example.com")repo = UserRepository()email_service = EmailService() repo.save(user)email_service.send_welcome_email(user)

2. Open/Closed Principle (OCP)

"Software entities should be open for extension but closed for modification."

Extend functionality mà không modify existing code.

❌ Violating OCP

class PaymentProcessor:    """Payment processor that needs modification for new payment methods."""        def process_payment(self, amount: float, method: str):        if method == "credit_card":            # Credit card logic            print(f"Processing ${amount} via credit card")        elif method == "paypal":            # PayPal logic            print(f"Processing ${amount} via PayPal")        elif method == "bitcoin":  # Need to modify for new method!            # Bitcoin logic            print(f"Processing ${amount} via Bitcoin")        else:            raise ValueError(f"Unknown payment method: {method}")

✅ Following OCP

from abc import ABC, abstractmethod class PaymentMethod(ABC):    """Abstract payment method."""        @abstractmethod    def process(self, amount: float):        """Process payment."""        pass class CreditCardPayment(PaymentMethod):    """Credit card payment implementation."""        def process(self, amount: float):        print(f"Processing ${amount} via credit card") class PayPalPayment(PaymentMethod):    """PayPal payment implementation."""        def process(self, amount: float):        print(f"Processing ${amount} via PayPal") class BitcoinPayment(PaymentMethod):    """Bitcoin payment implementation - NEW, no modification needed!"""        def process(self, amount: float):        print(f"Processing ${amount} via Bitcoin") class PaymentProcessor:    """Payment processor - closed for modification, open for extension."""        def process_payment(self, amount: float, method: PaymentMethod):        """Process payment using any payment method."""        method.process(amount) # Sử dụngprocessor = PaymentProcessor() # Existing methodsprocessor.process_payment(100, CreditCardPayment())processor.process_payment(200, PayPalPayment()) # New method - no changes to PaymentProcessor!processor.process_payment(300, BitcoinPayment())

3. Liskov Substitution Principle (LSP)

"Subtypes must be substitutable for their base types."

Child classes không được thay đổi behavior của parent class.

❌ Violating LSP

class Bird:    """Base bird class."""        def fly(self):        """All birds can fly."""        print("Flying...") class Sparrow(Bird):    """Sparrow can fly - OK."""    pass class Penguin(Bird):    """Penguin cannot fly - VIOLATES LSP!"""        def fly(self):        raise Exception("Penguins cannot fly!") # Problem: Cannot substitute Penguin for Birddef make_bird_fly(bird: Bird):    bird.fly() make_bird_fly(Sparrow())  # ✅ Worksmake_bird_fly(Penguin())  # ❌ Breaks!

✅ Following LSP

from abc import ABC, abstractmethod class Bird(ABC):    """Base bird class."""        @abstractmethod    def move(self):        """All birds can move."""        pass class FlyingBird(Bird):    """Birds that can fly."""        def move(self):        self.fly()        def fly(self):        print("Flying...") class FlightlessBird(Bird):    """Birds that cannot fly."""        def move(self):        self.walk()        def walk(self):        print("Walking...") class Sparrow(FlyingBird):    """Sparrow implementation."""    pass class Penguin(FlightlessBird):    """Penguin implementation."""    pass # Now substitutabledef make_bird_move(bird: Bird):    bird.move() make_bird_move(Sparrow())  # ✅ Flying...make_bird_move(Penguin())  # ✅ Walking...

4. Interface Segregation Principle (ISP)

"Clients should not be forced to depend on interfaces they don't use."

Chia interfaces thành smaller, specific ones.

❌ Violating ISP

from abc import ABC, abstractmethod class Worker(ABC):    """Fat interface - not all workers need all methods."""        @abstractmethod    def work(self):        pass        @abstractmethod    def eat(self):        pass        @abstractmethod    def sleep(self):        pass class Human(Worker):    """Human needs all methods - OK."""        def work(self):        print("Working...")        def eat(self):        print("Eating...")        def sleep(self):        print("Sleeping...") class Robot(Worker):    """Robot doesn't eat/sleep - FORCED to implement!"""        def work(self):        print("Working...")        def eat(self):        raise NotImplementedError("Robots don't eat")        def sleep(self):        raise NotImplementedError("Robots don't sleep")

✅ Following ISP

from abc import ABC, abstractmethod class Workable(ABC):    """Work interface."""        @abstractmethod    def work(self):        pass class Eatable(ABC):    """Eat interface."""        @abstractmethod    def eat(self):        pass class Sleepable(ABC):    """Sleep interface."""        @abstractmethod    def sleep(self):        pass class Human(Workable, Eatable, Sleepable):    """Human implements all interfaces."""        def work(self):        print("Working...")        def eat(self):        print("Eating...")        def sleep(self):        print("Sleeping...") class Robot(Workable):    """Robot only implements Workable."""        def work(self):        print("Working 24/7...") # Usagedef make_work(worker: Workable):    worker.work() def make_eat(eater: Eatable):    eater.eat() human = Human()robot = Robot() make_work(human)  # ✅make_work(robot)  # ✅make_eat(human)   # ✅# make_eat(robot) # ❌ Type error - Robot is not Eatable

5. Dependency Inversion Principle (DIP)

"Depend on abstractions, not on concretions."

High-level modules không nên depend on low-level modules.

❌ Violating DIP

class MySQLDatabase:    """Concrete MySQL implementation."""        def connect(self):        print("Connecting to MySQL...")        def query(self, sql: str):        print(f"Executing MySQL query: {sql}") class UserService:    """UserService depends on concrete MySQLDatabase."""        def __init__(self):        self.db = MySQLDatabase()  # Tight coupling!        def get_user(self, user_id: int):        self.db.connect()        self.db.query(f"SELECT * FROM users WHERE id = {user_id}") # Problem: Hard to switch databases or test

✅ Following DIP

from abc import ABC, abstractmethod class Database(ABC):    """Database abstraction."""        @abstractmethod    def connect(self):        pass        @abstractmethod    def query(self, sql: str):        pass class MySQLDatabase(Database):    """MySQL implementation."""        def connect(self):        print("Connecting to MySQL...")        def query(self, sql: str):        print(f"Executing MySQL query: {sql}") class PostgreSQLDatabase(Database):    """PostgreSQL implementation."""        def connect(self):        print("Connecting to PostgreSQL...")        def query(self, sql: str):        print(f"Executing PostgreSQL query: {sql}") class UserService:    """UserService depends on Database abstraction."""        def __init__(self, database: Database):        self.db = database  # Inject dependency!        def get_user(self, user_id: int):        self.db.connect()        self.db.query(f"SELECT * FROM users WHERE id = {user_id}") # Usage - easy to switch implementationsmysql_service = UserService(MySQLDatabase())postgres_service = UserService(PostgreSQLDatabase()) # Easy to test with mockclass MockDatabase(Database):    def connect(self):        pass        def query(self, sql: str):        return {"id": 1, "name": "Test"} test_service = UserService(MockDatabase())

Design Patterns

1. Singleton Pattern

Ensure một class chỉ có một instance duy nhất.

class Singleton:    """Singleton implementation using __new__."""        _instance = None        def __new__(cls):        if cls._instance is None:            cls._instance = super().__new__(cls)        return cls._instance # Thread-safe Singletonimport threading class ThreadSafeSingleton:    """Thread-safe singleton."""        _instance = None    _lock = threading.Lock()        def __new__(cls):        if cls._instance is None:            with cls._lock:                # Double-checked locking                if cls._instance is None:                    cls._instance = super().__new__(cls)        return cls._instance # Singleton decoratordef singleton(cls):    """Singleton decorator."""    instances = {}        def get_instance(*args, **kwargs):        if cls not in instances:            instances[cls] = cls(*args, **kwargs)        return instances[cls]        return get_instance @singletonclass DatabaseConnection:    """Database connection as singleton."""        def __init__(self):        print("Creating database connection...")        self.connection = "connected" # Usagedb1 = DatabaseConnection()db2 = DatabaseConnection()print(db1 is db2)  # True - same instance

2. Factory Pattern

Tạo objects mà không specify exact class.

from abc import ABC, abstractmethodfrom typing import Dict, Type class Animal(ABC):    """Abstract animal."""        @abstractmethod    def speak(self) -> str:        pass class Dog(Animal):    """Dog implementation."""        def speak(self) -> str:        return "Woof!" class Cat(Animal):    """Cat implementation."""        def speak(self) -> str:        return "Meow!" class Cow(Animal):    """Cow implementation."""        def speak(self) -> str:        return "Moo!" class AnimalFactory:    """Factory to create animals."""        _animals: Dict[str, Type[Animal]] = {        'dog': Dog,        'cat': Cat,        'cow': Cow,    }        @classmethod    def create_animal(cls, animal_type: str) -> Animal:        """Create animal by type."""        animal_class = cls._animals.get(animal_type.lower())                if not animal_class:            raise ValueError(f"Unknown animal type: {animal_type}")                return animal_class()        @classmethod    def register_animal(cls, animal_type: str, animal_class: Type[Animal]):        """Register new animal type."""        cls._animals[animal_type.lower()] = animal_class # Usagefactory = AnimalFactory() dog = factory.create_animal('dog')print(dog.speak())  # Woof! cat = factory.create_animal('cat')print(cat.speak())  # Meow! # Register new animal typeclass Bird(Animal):    def speak(self) -> str:        return "Chirp!" factory.register_animal('bird', Bird)bird = factory.create_animal('bird')print(bird.speak())  # Chirp!

3. Observer Pattern

Notify observers khi state changes.

from abc import ABC, abstractmethodfrom typing import List class Observer(ABC):    """Observer interface."""        @abstractmethod    def update(self, subject: 'Subject'):        """Called when subject changes."""        pass class Subject:    """Subject that notifies observers."""        def __init__(self):        self._observers: List[Observer] = []        self._state = None        def attach(self, observer: Observer):        """Attach observer."""        if observer not in self._observers:            self._observers.append(observer)        def detach(self, observer: Observer):        """Detach observer."""        self._observers.remove(observer)        def notify(self):        """Notify all observers."""        for observer in self._observers:            observer.update(self)        @property    def state(self):        return self._state        @state.setter    def state(self, value):        """Set state and notify observers."""        self._state = value        self.notify() class EmailObserver(Observer):    """Send email when subject changes."""        def update(self, subject: Subject):        print(f"Email: State changed to {subject.state}") class SMSObserver(Observer):    """Send SMS when subject changes."""        def update(self, subject: Subject):        print(f"SMS: State changed to {subject.state}") class LogObserver(Observer):    """Log when subject changes."""        def update(self, subject: Subject):        print(f"LOG: State changed to {subject.state}") # Usagesubject = Subject() email_observer = EmailObserver()sms_observer = SMSObserver()log_observer = LogObserver() subject.attach(email_observer)subject.attach(sms_observer)subject.attach(log_observer) # Change state - all observers notifiedsubject.state = "active"# Output:# Email: State changed to active# SMS: State changed to active# LOG: State changed to active # Detach observersubject.detach(sms_observer)subject.state = "inactive"# Output:# Email: State changed to inactive# LOG: State changed to inactive

4. Strategy Pattern

Encapsulate algorithms và make them interchangeable.

from abc import ABC, abstractmethod class SortStrategy(ABC):    """Strategy interface for sorting."""        @abstractmethod    def sort(self, data: list) -> list:        pass class BubbleSort(SortStrategy):    """Bubble sort implementation."""        def sort(self, data: list) -> list:        data = data.copy()        n = len(data)        for i in range(n):            for j in range(0, n-i-1):                if data[j] > data[j+1]:                    data[j], data[j+1] = data[j+1], data[j]        return data class QuickSort(SortStrategy):    """Quick sort implementation."""        def sort(self, data: list) -> list:        if len(data) <= 1:            return data                pivot = data[len(data) // 2]        left = [x for x in data if x < pivot]        middle = [x for x in data if x == pivot]        right = [x for x in data if x > pivot]                return self.sort(left) + middle + self.sort(right) class MergeSort(SortStrategy):    """Merge sort implementation."""        def sort(self, data: list) -> list:        if len(data) <= 1:            return data                mid = len(data) // 2        left = self.sort(data[:mid])        right = self.sort(data[mid:])                return self._merge(left, right)        def _merge(self, left: list, right: list) -> list:        result = []        i = j = 0                while i < len(left) and j < len(right):            if left[i] <= right[j]:                result.append(left[i])                i += 1            else:                result.append(right[j])                j += 1                result.extend(left[i:])        result.extend(right[j:])        return result class Sorter:    """Context that uses sorting strategy."""        def __init__(self, strategy: SortStrategy):        self._strategy = strategy        def set_strategy(self, strategy: SortStrategy):        """Change sorting strategy at runtime."""        self._strategy = strategy        def sort(self, data: list) -> list:        """Sort data using current strategy."""        return self._strategy.sort(data) # Usagedata = [64, 34, 25, 12, 22, 11, 90] sorter = Sorter(BubbleSort())print(sorter.sort(data))  # Bubble sort sorter.set_strategy(QuickSort())print(sorter.sort(data))  # Quick sort sorter.set_strategy(MergeSort())print(sorter.sort(data))  # Merge sort

5. Decorator Pattern

Add behavior to objects dynamically.

from abc import ABC, abstractmethod class Coffee(ABC):    """Coffee interface."""        @abstractmethod    def cost(self) -> float:        pass        @abstractmethod    def description(self) -> str:        pass class SimpleCoffee(Coffee):    """Basic coffee."""        def cost(self) -> float:        return 2.0        def description(self) -> str:        return "Simple Coffee" class CoffeeDecorator(Coffee):    """Base decorator."""        def __init__(self, coffee: Coffee):        self._coffee = coffee        def cost(self) -> float:        return self._coffee.cost()        def description(self) -> str:        return self._coffee.description() class MilkDecorator(CoffeeDecorator):    """Add milk."""        def cost(self) -> float:        return self._coffee.cost() + 0.5        def description(self) -> str:        return self._coffee.description() + ", Milk" class SugarDecorator(CoffeeDecorator):    """Add sugar."""        def cost(self) -> float:        return self._coffee.cost() + 0.2        def description(self) -> str:        return self._coffee.description() + ", Sugar" class VanillaDecorator(CoffeeDecorator):    """Add vanilla."""        def cost(self) -> float:        return self._coffee.cost() + 0.7        def description(self) -> str:        return self._coffee.description() + ", Vanilla" # Usagecoffee = SimpleCoffee()print(f"{coffee.description()}: ${coffee.cost()}")# Simple Coffee: $2.0 # Add milkcoffee = MilkDecorator(coffee)print(f"{coffee.description()}: ${coffee.cost()}")# Simple Coffee, Milk: $2.5 # Add sugarcoffee = SugarDecorator(coffee)print(f"{coffee.description()}: ${coffee.cost()}")# Simple Coffee, Milk, Sugar: $2.7 # Add vanillacoffee = VanillaDecorator(coffee)print(f"{coffee.description()}: ${coffee.cost()}")# Simple Coffee, Milk, Sugar, Vanilla: $3.4

2 Ứng Dụng Thực Tế

1. Plugin System với Factory và Strategy

from abc import ABC, abstractmethodfrom typing import Dict, Type, Any class Plugin(ABC):    """Plugin interface."""        @abstractmethod    def execute(self, data: Any) -> Any:        """Execute plugin logic."""        pass        @abstractmethod    def get_name(self) -> str:        """Get plugin name."""        pass class DataValidationPlugin(Plugin):    """Data validation plugin."""        def execute(self, data: Dict) -> Dict:        # Validate data        if not data.get('email'):            raise ValueError("Email is required")        return data        def get_name(self) -> str:        return "data_validation" class DataTransformPlugin(Plugin):    """Data transformation plugin."""        def execute(self, data: Dict) -> Dict:        # Transform data        data['email'] = data['email'].lower()        return data        def get_name(self) -> str:        return "data_transform" class DataEnrichmentPlugin(Plugin):    """Data enrichment plugin."""        def execute(self, data: Dict) -> Dict:        # Enrich data        data['processed_at'] = "2024-01-01"        return data        def get_name(self) -> str:        return "data_enrichment" class PluginFactory:    """Factory to create and manage plugins."""        _plugins: Dict[str, Type[Plugin]] = {}        @classmethod    def register(cls, plugin_class: Type[Plugin]):        """Register plugin."""        plugin = plugin_class()        cls._plugins[plugin.get_name()] = plugin_class        @classmethod    def create(cls, plugin_name: str) -> Plugin:        """Create plugin instance."""        plugin_class = cls._plugins.get(plugin_name)        if not plugin_class:            raise ValueError(f"Unknown plugin: {plugin_name}")        return plugin_class()        @classmethod    def get_available_plugins(cls) -> list:        """Get list of available plugins."""        return list(cls._plugins.keys()) class PluginPipeline:    """Execute plugins in sequence."""        def __init__(self):        self.plugins: list[Plugin] = []        def add_plugin(self, plugin: Plugin):        """Add plugin to pipeline."""        self.plugins.append(plugin)        def execute(self, data: Any) -> Any:        """Execute all plugins in order."""        result = data        for plugin in self.plugins:            print(f"Executing plugin: {plugin.get_name()}")            result = plugin.execute(result)        return result # Register pluginsPluginFactory.register(DataValidationPlugin)PluginFactory.register(DataTransformPlugin)PluginFactory.register(DataEnrichmentPlugin) # Usagepipeline = PluginPipeline() # Add plugins dynamicallyfor plugin_name in ['data_validation', 'data_transform', 'data_enrichment']:    plugin = PluginFactory.create(plugin_name)    pipeline.add_plugin(plugin) # Process datadata = {'email': 'USER@EXAMPLE.COM'}result = pipeline.execute(data)print(result)# {'email': 'user@example.com', 'processed_at': '2024-01-01'}

2. Event-Driven Architecture với Observer

from abc import ABC, abstractmethodfrom typing import Dict, List, Anyfrom datetime import datetime class Event:    """Event data."""        def __init__(self, event_type: str, data: Dict[str, Any]):        self.type = event_type        self.data = data        self.timestamp = datetime.now() class EventHandler(ABC):    """Event handler interface."""        @abstractmethod    def handle(self, event: Event):        """Handle event."""        pass        @abstractmethod    def can_handle(self, event_type: str) -> bool:        """Check if handler can handle event type."""        pass class UserCreatedHandler(EventHandler):    """Handle user created events."""        def handle(self, event: Event):        print(f"User created: {event.data['username']}")        # Send welcome email, create profile, etc.        def can_handle(self, event_type: str) -> bool:        return event_type == "user.created" class UserUpdatedHandler(EventHandler):    """Handle user updated events."""        def handle(self, event: Event):        print(f"User updated: {event.data['username']}")        # Update cache, notify watchers, etc.        def can_handle(self, event_type: str) -> bool:        return event_type == "user.updated" class OrderPlacedHandler(EventHandler):    """Handle order placed events."""        def handle(self, event: Event):        print(f"Order placed: #{event.data['order_id']}")        # Process payment, send confirmation, update inventory        def can_handle(self, event_type: str) -> bool:        return event_type == "order.placed" class EventBus:    """Event bus for publishing and subscribing to events."""        def __init__(self):        self._handlers: List[EventHandler] = []        self._event_log: List[Event] = []        def subscribe(self, handler: EventHandler):        """Subscribe handler to events."""        self._handlers.append(handler)        def publish(self, event: Event):        """Publish event to all relevant handlers."""        print(f"\n[EventBus] Publishing event: {event.type}")                # Log event        self._event_log.append(event)                # Notify handlers        handled = False        for handler in self._handlers:            if handler.can_handle(event.type):                handler.handle(event)                handled = True                if not handled:            print(f"[EventBus] No handlers for event: {event.type}")        def get_event_log(self) -> List[Event]:        """Get event history."""        return self._event_log # Usageevent_bus = EventBus() # Subscribe handlersevent_bus.subscribe(UserCreatedHandler())event_bus.subscribe(UserUpdatedHandler())event_bus.subscribe(OrderPlacedHandler()) # Publish eventsevent_bus.publish(Event("user.created", {    "username": "john_doe",    "email": "john@example.com"})) event_bus.publish(Event("order.placed", {    "order_id": "12345",    "total": 99.99})) event_bus.publish(Event("user.updated", {    "username": "john_doe",    "email": "newemail@example.com"})) # View event logprint(f"\nTotal events: {len(event_bus.get_event_log())}")

Best Practices Summary

Code Quality Checklist

# ✅ Follow PEP 8# ✅ Use type hints# ✅ Write docstrings# ✅ Keep functions small (< 50 lines)# ✅ Single responsibility per class# ✅ Dependency injection# ✅ Write tests# ✅ Handle exceptions properly# ✅ Use meaningful names# ✅ Don't repeat yourself (DRY)# ✅ Keep it simple (KISS)# ✅ You aren't gonna need it (YAGNI) def good_function(user_id: int, email: str) -> bool:    """    Send welcome email to user.        Args:        user_id: User's unique identifier        email: User's email address        Returns:        True if email sent successfully, False otherwise        Raises:        ValueError: If email format is invalid    """    if not _is_valid_email(email):        raise ValueError(f"Invalid email: {email}")        try:        _send_email(email, "Welcome!")        return True    except EmailError as e:        logger.error(f"Failed to send email: {e}")        return False

Bài Tập Thực Hành

Bài 1: Refactor with SOLID

Refactor một class lớn để follow SOLID principles.

Bài 2: Implement Singleton

Tạo thread-safe Singleton cho database connection.

Bài 3: Factory Pattern

Implement factory pattern cho notification system (email, SMS, push).

Bài 4: Observer Pattern

Tạo event system với multiple observers.

Bài 5: Strategy Pattern

Implement strategy pattern cho data export (CSV, JSON, XML).

Tóm Tắt

Trong Part 2 chúng ta đã học:

✅ SOLID Principles:
- Single Responsibility (SRP)
- Open/Closed (OCP)
- Liskov Substitution (LSP)
- Interface Segregation (ISP)
- Dependency Inversion (DIP)
✅ Design Patterns:
- Singleton (single instance)
- Factory (object creation)
- Observer (event notification)
- Strategy (interchangeable algorithms)
- Decorator (add behavior dynamically)
✅ Real Applications:
- Plugin system
- Event-driven architecture
✅ Best Practices Checklist

Best practices giúp code maintainable, testable, và scalable! 🚀

Bài tiếp theo: Bài 20: Working with APIs - Học cách làm việc với REST APIs! 🌐

#design-patterns

Python Best Practices

Python Best Practices (Phần 2)