class Book:
    def __init__(self, title, author, pages):
        self.title = title
        self.author = author
        self.pages = pages
    
    def __str__(self):
        return f"'{self.title}' by {self.author}"
    
    def __repr__(self):
        return f"Book(title='{self.title}', author='{self.author}')"
    
    def __len__(self):
        return self.pages

book = Book("Python Crash Course", "Eric Matthes", 544)
print(book)        # 'Python Crash Course' by Eric Matthes
print(repr(book))  # Book(title='Python Crash Course', author='Eric Matthes')
print(len(book))   # 544

class Box:
    def __init__(self, weight):
        self.weight = weight
    
    def __lt__(self, other):
        return self.weight < other.weight
    
    def __eq__(self, other):
        return self.weight == other.weight

box1 = Box(10)
box2 = Box(20)
print(box1 < box2)  # True
print(box1 == box2) # False

class Vector:
    def __init__(self, x, y):
        self.x = x
        self.y = y
    
    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)
    
    def __str__(self):
        return f"Vector({self.x}, {self.y})"

v1 = Vector(1, 2)
v2 = Vector(3, 4)
print(v1 + v2)  # Vector(4, 6)

class Countdown:
    def __init__(self, start):
        self.start = start
    
    def __iter__(self):
        return self
    
    def __next__(self):
        if self.start <= 0:
            raise StopIteration
        self.start -= 1
        return self.start + 1

for i in Countdown(5):
    print(i, end=' ')  # 5 4 3 2 1

class SparseList:
    def __init__(self, size):
        self.size = size
        self.data = {}
    
    def __getitem__(self, index):
        if index < 0 or index >= self.size:
            raise IndexError("Index out of range")
        return self.data.get(index, 0)
    
    def __setitem__(self, index, value):
        if index < 0 or index >= self.size:
            raise IndexError("Index out of range")
        self.data[index] = value
    
    def __contains__(self, value):
        return value in self.data.values()

sparse = SparseList(10)
sparse[3] = 42
print(sparse[3])      # 42
print(42 in sparse)   # True
print(sparse[4])      # 0

class ProtectedAttributes:
    def __init__(self):
        self._protected = "This is protected"
    
    def __getattr__(self, name):
        return f"'{name}' attribute not found"
    
    def __setattr__(self, name, value):
        if name.startswith('_'):
            super().__setattr__(name, value)
        else:
            raise AttributeError(f"Cannot set attribute '{name}'")

obj = ProtectedAttributes()
print(obj.nonexistent)  # 'nonexistent' attribute not found
# obj.public = "test"   # Raises AttributeError

class DatabaseConnection:
    def __enter__(self):
        print("Connecting to database...")
        return self
    
    def __exit__(self, exc_type, exc_val, exc_tb):
        print("Closing connection...")
        if exc_type:
            print(f"Error occurred: {exc_val}")
        return True  # Suppress exceptions

with DatabaseConnection() as conn:
    print("Executing query...")
    # raise ValueError("Invalid SQL")  # Would be handled by __exit__

class Counter:
    def __init__(self):
        self.count = 0
    
    def __call__(self, increment=1):
        self.count += increment
        return self.count

counter = Counter()
print(counter())      # 1
print(counter(5))     # 6

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

class SlottedPoint:
    __slots__ = ['x', 'y']
    def __init__(self, x, y):
        self.x = x
        self.y = y

import sys
regular = RegularPoint(1, 2)
slotted = SlottedPoint(1, 2)
print(sys.getsizeof(regular))  # 56 bytes (approx)
print(sys.getsizeof(slotted))  # 32 bytes (approx)

class Fraction:
    def __init__(self, numerator, denominator):
        self.numerator = numerator
        self.denominator = denominator
    
    def __add__(self, other):
        new_num = self.numerator * other.denominator + other.numerator * self.denominator
        new_den = self.denominator * other.denominator
        return Fraction(new_num, new_den)
    
    def __str__(self):
        return f"{self.numerator}/{self.denominator}"

f1 = Fraction(1, 2)
f2 = Fraction(1, 3)
print(f1 + f2)  # 5/6

class LazyProperty:
    def __init__(self, func):
        self.func = func
        self.name = func.__name__
    
    def __get__(self, obj, type=None):
        if obj is None:
            return self
        value = self.func(obj)
        setattr(obj, self.name, value)
        return value

class Circle:
    def __init__(self, radius):
        self.radius = radius
    
    @LazyProperty
    def area(self):
        print("Computing area...")
        return 3.14 * self.radius ** 2

c = Circle(5)
print(c.area)  # First call: computes and caches
print(c.area)  # Subsequent call: returns cached value

# 好的實(shí)踐示例
class GoodExample:
    """遵循最佳實(shí)踐的類實(shí)現(xiàn)"""
    
    def __init__(self, value):
        self._value = value
    
    def __repr__(self):
        return f"{self.__class__.__name__}({self._value!r})"
    
    def __str__(self):
        return f"Value: {self._value}"
    
    def __eq__(self, other):
        if not isinstance(other, GoodExample):
            return NotImplemented
        return self._value == other._value
    
    def __hash__(self):
        return hash(self._value)