@property

⋯3class BankAccount:4    """Bank account with validation."""56    def __init__(self, account_number, balance=0): #?init_account7        self._account_number = account_number #?store_account_number8        self._balance = balance #?store_balance910    @property #?balance_property11    def balance(self): #?balance_getter12        """Get current balance."""13        return self._balance #?return_balance1415    @balance.setter #?balance_setter16    def balance(self, value): #?balance_setter_method17        """Set balance with validation."""18        if value < 0: #?check_negative19            raise ValueError("Balance cannot be negative") #?raise_negative_balance20        self._balance = value #?store_valid_balance212223class User:24    """User with multiple validated properties."""⋯

⋯72def main():73    print("=== Properties with Validation ===\n")7475    # Bank account validation #?bank_demo76    print("--- Bank Account ---")77    account = BankAccount("12345", 1000) #?create_account78    print(f"Initial balance: ${account.balance}") #?print_initial⋯

⋯6def __init__(self⟨BankAccount A⟩, account_number12345, balance1000=0): #?init_account7    self._account_number→ 12345 = account_number12345 #?store_account_number8    self._balance→ 1000 = balance1000 #?store_balance⋯

⋯76print("--- Bank Account ---")77account→ ⟨BankAccount A⟩ = BankAccount("12345", 1000) #?create_account78print(f"Initial balance: ${account.balance1000}") #?print_initial⋯

⋯10@property #?balance_property11def balance(self⟨BankAccount A⟩): #?balance_getter12    """Get current balance."""13    return self._balance1000 #?return_balance⋯

⋯77account = BankAccount("12345", 1000) #?create_account78print(f"Initial balance: ${account.balance1000}") #?print_initial7980account.balance = 1500 #?set_valid_balance81print(f"After deposit: ${account.balance}") #?print_after_deposit⋯

⋯15@balance.setter #?balance_setter16def balance(self⟨BankAccount A⟩, value1500): #?balance_setter_method17    """Set balance with validation."""18    if value < 0: #?check_negative19        raise ValueError("Balance cannot be negative") #?raise_negative_balance20    self._balance→ 1500 = value1500 #?store_valid_balance⋯

⋯80account.balance→ 1500 = 1500 #?set_valid_balance81print(f"After deposit: ${account.balance1500}") #?print_after_deposit⋯

⋯10@property #?balance_property11def balance(self⟨BankAccount A⟩): #?balance_getter12    """Get current balance."""13    return self._balance1500 #?return_balance⋯

⋯80account.balance = 1500 #?set_valid_balance81print(f"After deposit: ${account.balance1500}") #?print_after_deposit⋯

⋯15@balance.setter #?balance_setter16def balance(self⟨BankAccount A⟩, value-100): #?balance_setter_method17    """Set balance with validation."""18    if value < 0: #?check_negative⋯

⋯17"""Set balance with validation."""18if value-100 < 0: #?check_negative19    raise ValueError("Balance cannot be negative") #?raise_negative_balance20self._balance = value #?store_valid_balance⋯

⋯84    account.balance = -100 #?set_negative_balance85except ValueError as e: #?catch_balance_error86    print(f"Error: {eBalance cannot be negative}") #?print_balance_error8788# User validation #?user_demo⋯

⋯88# User validation #?user_demo89print("\n--- User Validation ---")⋯

⋯26def __init__(self⟨User B⟩, usernamealice, emailalice@example.com, age25): #?init_user27    self.username = usernamealice  # Uses setter28    self.email = email  # Uses setter⋯

⋯26def __init__(self, username, email, age): #?init_user27    self.username→ alice = usernamealice  # Uses setter28    self.email = emailalice@example.com  # Uses setter29    self.age = age  # Uses setter3031@property #?username_property32def username(self): #?username_getter33    """Get username."""34    return self._username #?return_username3536@username.setter #?username_setter37def username(self⟨User B⟩, valuealice): #?username_setter_method38    """Set username - must be 3-20 chars."""39    if not isinstance(value, str): #?check_string40        raise TypeError("Username must be a string") #?raise_type41    if not (3 <= len(value) <= 20): #?check_length42        raise ValueError("Username must be 3-20 characters") #?raise_length43    self._username→ alice = valuealice #?store_username⋯

⋯27    self.username = username  # Uses setter28    self.email→ alice@example.com = emailalice@example.com  # Uses setter29    self.age = age25  # Uses setter3031@property #?username_property32def username(self): #?username_getter33    """Get username."""34    return self._username #?return_username3536@username.setter #?username_setter37def username(self, value): #?username_setter_method38    """Set username - must be 3-20 chars."""39    if not isinstance(value, str): #?check_string40        raise TypeError("Username must be a string") #?raise_type41    if not (3 <= len(value) <= 20): #?check_length42        raise ValueError("Username must be 3-20 characters") #?raise_length43    self._username = value #?store_username4445@property #?email_property46def email(self): #?email_getter47    """Get email."""48    return self._email #?return_email4950@email.setter #?email_setter51def email(self⟨User B⟩, valuealice@example.com): #?email_setter_method52    """Set email - must contain @."""53    if "@" not in value: #?check_at54        raise ValueError("Invalid email format") #?raise_email55    self._email→ alice@example.com = valuealice@example.com #?store_email⋯

⋯28    self.email = email  # Uses setter29    self.age→ 25 = age25  # Uses setter3031@property #?username_property32def username(self): #?username_getter33    """Get username."""34    return self._username #?return_username3536@username.setter #?username_setter37def username(self, value): #?username_setter_method38    """Set username - must be 3-20 chars."""39    if not isinstance(value, str): #?check_string40        raise TypeError("Username must be a string") #?raise_type41    if not (3 <= len(value) <= 20): #?check_length42        raise ValueError("Username must be 3-20 characters") #?raise_length43    self._username = value #?store_username4445@property #?email_property46def email(self): #?email_getter47    """Get email."""48    return self._email #?return_email4950@email.setter #?email_setter51def email(self, value): #?email_setter_method52    """Set email - must contain @."""53    if "@" not in value: #?check_at54        raise ValueError("Invalid email format") #?raise_email55    self._email = value #?store_email5657@property #?age_property58def age(self): #?age_getter59    """Get age."""60    return self._age #?return_age6162@age.setter #?age_setter63def age(self⟨User B⟩, value25): #?age_setter_method64    """Set age - must be 0-150."""65    if not isinstance(value, int): #?check_int66        raise TypeError("Age must be an integer") #?raise_age_type67    if not (0 <= value <= 150): #?check_age_range68        raise ValueError("Age must be between 0 and 150") #?raise_age_range69    self._age→ 25 = value25 #?store_age⋯

⋯91try: #?try_create_user92    user→ ⟨User B⟩ = User("alice", "alice@example.com", 25) #?create_valid_user93    print(f"Created user: {user.usernamealice}, {user.emailalice@example.com}, {user.age25}") #?print_user94except (ValueError, TypeError) as e: #?catch_create_error⋯

⋯31@property #?username_property32def username(self⟨User B⟩): #?username_getter33    """Get username."""34    return self._usernamealice #?return_username⋯

⋯45@property #?email_property46def email(self⟨User B⟩): #?email_getter47    """Get email."""48    return self._emailalice@example.com #?return_email⋯

⋯57@property #?age_property58def age(self⟨User B⟩): #?age_getter59    """Get age."""60    return self._age25 #?return_age⋯

⋯92    user = User("alice", "alice@example.com", 25) #?create_valid_user93    print(f"Created user: {user.usernamealice}, {user.emailalice@example.com}, {user.age25}") #?print_user94except (ValueError, TypeError) as e: #?catch_create_error⋯

⋯97# Invalid username #?invalid_username98print("\n--- Invalid Username ---")99try: #?try_invalid_username⋯

⋯40    raise TypeError("Username must be a string") #?raise_type41if not (3 <= len(valueab) <= 20): #?check_length42    raise ValueError("Username must be 3-20 characters") #?raise_length43self._username = value #?store_username⋯

⋯100    user.username = "ab"  # Too short101except ValueError as e: #?catch_username_error102    print(f"Error: {eUsername must be 3-20 characters}") #?print_username_error103104# Invalid email #?invalid_email⋯

⋯104# Invalid email #?invalid_email105print("\n--- Invalid Email ---")106try: #?try_invalid_email⋯

⋯50@email.setter #?email_setter51def email(self⟨User B⟩, valuenot-an-email): #?email_setter_method52    """Set email - must contain @."""53    if "@" not in value: #?check_at⋯

⋯52"""Set email - must contain @."""53if "@" not in valuenot-an-email: #?check_at54    raise ValueError("Invalid email format") #?raise_email55self._email = value #?store_email⋯

⋯107    user.email = "not-an-email" #?set_invalid_email108except ValueError as e: #?catch_email_error109    print(f"Error: {eInvalid email format}") #?print_email_error110111# Invalid age type #?invalid_age_type⋯

⋯111# Invalid age type #?invalid_age_type112print("\n--- Invalid Age Type ---")113try: #?try_invalid_age_type⋯

⋯64"""Set age - must be 0-150."""65if not isinstance(value25, int): #?check_int66    raise TypeError("Age must be an integer") #?raise_age_type67if not (0 <= value <= 150): #?check_age_range⋯

⋯114    user.age = "25"  # String instead of int115except TypeError as e: #?catch_age_type_error116    print(f"Error: {eAge must be an integer}") #?print_age_type_error117118# Invalid age range #?invalid_age_range⋯

⋯118# Invalid age range #?invalid_age_range119print("\n--- Invalid Age Range ---")120try: #?try_invalid_age_range⋯

⋯66    raise TypeError("Age must be an integer") #?raise_age_type67if not (0 <= value200 <= 150): #?check_age_range68    raise ValueError("Age must be between 0 and 150") #?raise_age_range69self._age = value #?store_age⋯

⋯121    user.age = 200 #?set_out_of_range_age122except ValueError as e: #?catch_age_range_error123    print(f"Error: {eAge must be between 0 and 150}") #?print_age_range_error124125# Validation during construction #?construction_validation⋯

⋯125# Validation during construction #?construction_validation126print("\n--- Validation During Construction ---")127try: #?try_invalid_construction⋯

⋯26def __init__(self⟨User C⟩, usernamea, emailbad, age300): #?init_user27    self.username = usernamea  # Uses setter28    self.email = email  # Uses setter⋯

⋯40    raise TypeError("Username must be a string") #?raise_type41if not (3 <= len(valuea) <= 20): #?check_length42    raise ValueError("Username must be 3-20 characters") #?raise_length43self._username = value #?store_username⋯

⋯128    invalid_user = User("a", "bad", 300) #?create_invalid_user129except (ValueError, TypeError) as e: #?catch_construction_error130    print(f"Construction failed: {eUsername must be 3-20 characters}") #?print_construction_error131132print("\n=== Key Points ===")⋯

⋯132    print("\n=== Key Points ===")133    print("""1341. Properties enable validation on assignment1352. Validation runs even during __init__1363. Can check type, range, format, etc.1374. Raises appropriate exceptions1385. Keeps object in valid state139    """)⋯

⋯142if __name__ == "__main__":143    main()⋯

⋯6class Circle:7    """Circle with computed properties."""89    def __init__(self, radius): #?init_circle10        self._radius = radius #?store_radius1112    @property #?radius_property13    def radius(self): #?radius_getter14        """Get radius."""15        return self._radius #?return_radius1617    @radius.setter #?radius_setter18    def radius(self, value): #?radius_setter_method19        """Set radius."""20        if value <= 0: #?check_positive21            raise ValueError("Radius must be positive") #?raise_radius22        self._radius = value #?store_valid_radius2324    @property #?diameter_property25    def diameter(self): #?diameter_getter26        """Computed: diameter = 2 * radius."""27        return 2 * self._radius #?calc_diameter2829    @diameter.setter #?diameter_setter30    def diameter(self, value): #?diameter_setter_method31        """Set diameter (updates radius)."""32        self.radius = value / 2  # Uses radius setter3334    @property #?area_property35    def area(self): #?area_getter36        """Computed: area = π * r²."""37        return math.pi * self._radius ** 2 #?calc_area3839    @property #?circumference_property40    def circumference(self): #?circumference_getter41        """Computed: circumference = 2 * π * r."""42        return 2 * math.pi * self._radius #?calc_circumference434445class Temperature:46    """Temperature with Celsius/Fahrenheit conversion."""4748    def __init__(self, celsius=0): #?init_temp49        self._celsius = celsius #?store_celsius5051    @property #?celsius_property52    def celsius(self): #?celsius_getter53        """Get temperature in Celsius."""54        return self._celsius #?return_celsius5556    @celsius.setter #?celsius_setter57    def celsius(self, value): #?celsius_setter_method58        """Set temperature in Celsius."""59        if value < -273.15: #?check_absolute_zero60            raise ValueError("Below absolute zero!") #?raise_absolute_zero61        self._celsius = value #?store_valid_celsius6263    @property #?fahrenheit_property64    def fahrenheit(self): #?fahrenheit_getter65        """Computed: convert to Fahrenheit."""66        return self._celsius * 9/5 + 32 #?calc_fahrenheit6768    @fahrenheit.setter #?fahrenheit_setter69    def fahrenheit(self, value): #?fahrenheit_setter_method70        """Set temperature in Fahrenheit (updates celsius)."""71        self.celsius = (value - 32) * 5/9 #?set_celsius_from_fahrenheit7273    @property #?kelvin_property74    def kelvin(self): #?kelvin_getter75        """Computed: convert to Kelvin."""76        return self._celsius + 273.15 #?calc_kelvin7778    @kelvin.setter #?kelvin_setter79    def kelvin(self, value): #?kelvin_setter_method80        """Set temperature in Kelvin (updates celsius)."""81        self.celsius = value - 273.15 #?set_celsius_from_kelvin828384class Rectangle:85    """Rectangle with computed properties."""⋯

⋯132def main():133    print("=== Computed Properties ===\n")134135    # Circle with computed properties #?circle_demo136    print("--- Circle ---")137    circle = Circle(5) #?create_circle138    print(f"Radius: {circle.radius}") #?print_radius⋯

⋯9def __init__(self⟨Circle A⟩, radius5): #?init_circle10    self._radius→ 5 = radius5 #?store_radius⋯

⋯136print("--- Circle ---")137circle→ ⟨Circle A⟩ = Circle(5) #?create_circle138print(f"Radius: {circle.radius5}") #?print_radius139print(f"Diameter: {circle.diameter}") #?print_diameter⋯

⋯12@property #?radius_property13def radius(self⟨Circle A⟩): #?radius_getter14    """Get radius."""15    return self._radius5 #?return_radius⋯

⋯137circle = Circle(5) #?create_circle138print(f"Radius: {circle.radius5}") #?print_radius139print(f"Diameter: {circle.diameter10}") #?print_diameter140print(f"Area: {circle.area:.2f}") #?print_area⋯

⋯24@property #?diameter_property25def diameter(self⟨Circle A⟩): #?diameter_getter26    """Computed: diameter = 2 * radius."""27    return 2 * self._radius5 #?calc_diameter⋯

⋯138print(f"Radius: {circle.radius}") #?print_radius139print(f"Diameter: {circle.diameter10}") #?print_diameter140print(f"Area: {circle.area78.53981633974483:.2f}") #?print_area141print(f"Circumference: {circle.circumference:.2f}") #?print_circumference⋯

⋯34@property #?area_property35def area(self⟨Circle A⟩): #?area_getter36    """Computed: area = π * r²."""37    return math.pi3.141592653589793 * self._radius5 ** 2 #?calc_area⋯

⋯139print(f"Diameter: {circle.diameter}") #?print_diameter140print(f"Area: {circle.area78.53981633974483:.2f}") #?print_area141print(f"Circumference: {circle.circumference31.41592653589793:.2f}") #?print_circumference⋯

⋯39@property #?circumference_property40def circumference(self⟨Circle A⟩): #?circumference_getter41    """Computed: circumference = 2 * π * r."""42    return 2 * math.pi3.141592653589793 * self._radius5 #?calc_circumference⋯

⋯140print(f"Area: {circle.area:.2f}") #?print_area141print(f"Circumference: {circle.circumference31.41592653589793:.2f}") #?print_circumference142143# Update via diameter #?update_diameter144print("\n--- Update via Diameter ---")145circle.diameter = 20 #?set_diameter146print(f"New radius: {circle.radius}") #?print_new_radius⋯

⋯29@diameter.setter #?diameter_setter30def diameter(self⟨Circle A⟩, value20): #?diameter_setter_method31    """Set diameter (updates radius)."""32    self.radius = value20 / 2  # Uses radius setter⋯

⋯17@radius.setter #?radius_setter18def radius(self⟨Circle A⟩, value10.0): #?radius_setter_method19    """Set radius."""20    if value <= 0: #?check_positive21        raise ValueError("Radius must be positive") #?raise_radius22    self._radius→ 10.0 = value10.0 #?store_valid_radius⋯

⋯31"""Set diameter (updates radius)."""32self.radius→ 10.0 = value20 / 2  # Uses radius setter⋯

⋯144print("\n--- Update via Diameter ---")145circle.diameter→ 20.0 = 20 #?set_diameter146print(f"New radius: {circle.radius10.0}") #?print_new_radius147print(f"New area: {circle.area:.2f}") #?print_new_area⋯

⋯12@property #?radius_property13def radius(self⟨Circle A⟩): #?radius_getter14    """Get radius."""15    return self._radius10.0 #?return_radius⋯

⋯145circle.diameter = 20 #?set_diameter146print(f"New radius: {circle.radius10.0}") #?print_new_radius147print(f"New area: {circle.area314.1592653589793:.2f}") #?print_new_area⋯

⋯34@property #?area_property35def area(self⟨Circle A⟩): #?area_getter36    """Computed: area = π * r²."""37    return math.pi3.141592653589793 * self._radius10.0 ** 2 #?calc_area⋯

⋯146print(f"New radius: {circle.radius}") #?print_new_radius147print(f"New area: {circle.area314.1592653589793:.2f}") #?print_new_area148149# Temperature conversions #?temp_demo150print("\n--- Temperature Conversions ---")151temp = Temperature(25) #?create_temp152print(f"Celsius: {temp.celsius}°C") #?print_celsius⋯

⋯48def __init__(self⟨Temperature B⟩, celsius25=0): #?init_temp49    self._celsius→ 25 = celsius25 #?store_celsius⋯

⋯150print("\n--- Temperature Conversions ---")151temp→ ⟨Temperature B⟩ = Temperature(25) #?create_temp152print(f"Celsius: {temp.celsius25}°C") #?print_celsius153print(f"Fahrenheit: {temp.fahrenheit}°F") #?print_fahrenheit⋯

⋯51@property #?celsius_property52def celsius(self⟨Temperature B⟩): #?celsius_getter53    """Get temperature in Celsius."""54    return self._celsius25 #?return_celsius⋯

⋯151temp = Temperature(25) #?create_temp152print(f"Celsius: {temp.celsius25}°C") #?print_celsius153print(f"Fahrenheit: {temp.fahrenheit77.0}°F") #?print_fahrenheit154print(f"Kelvin: {temp.kelvin}K") #?print_kelvin⋯

⋯63@property #?fahrenheit_property64def fahrenheit(self⟨Temperature B⟩): #?fahrenheit_getter65    """Computed: convert to Fahrenheit."""66    return self._celsius25 * 9/5 + 32 #?calc_fahrenheit⋯

⋯152print(f"Celsius: {temp.celsius}°C") #?print_celsius153print(f"Fahrenheit: {temp.fahrenheit77.0}°F") #?print_fahrenheit154print(f"Kelvin: {temp.kelvin298.15}K") #?print_kelvin⋯

⋯73@property #?kelvin_property74def kelvin(self⟨Temperature B⟩): #?kelvin_getter75    """Computed: convert to Kelvin."""76    return self._celsius25 + 273.15 #?calc_kelvin⋯

⋯153print(f"Fahrenheit: {temp.fahrenheit}°F") #?print_fahrenheit154print(f"Kelvin: {temp.kelvin298.15}K") #?print_kelvin155156# Set via Fahrenheit #?set_fahrenheit157print("\n--- Set via Fahrenheit ---")158temp.fahrenheit = 212 #?set_boiling159print(f"Celsius: {temp.celsius}°C") #?print_boiling_celsius⋯

⋯68@fahrenheit.setter #?fahrenheit_setter69def fahrenheit(self⟨Temperature B⟩, value212): #?fahrenheit_setter_method70    """Set temperature in Fahrenheit (updates celsius)."""71    self.celsius = (value212 - 32) * 5/9 #?set_celsius_from_fahrenheit⋯

⋯56@celsius.setter #?celsius_setter57def celsius(self⟨Temperature B⟩, value100.0): #?celsius_setter_method58    """Set temperature in Celsius."""59    if value < -273.15: #?check_absolute_zero60        raise ValueError("Below absolute zero!") #?raise_absolute_zero61    self._celsius→ 100.0 = value100.0 #?store_valid_celsius⋯

⋯70"""Set temperature in Fahrenheit (updates celsius)."""71self.celsius→ 100.0 = (value212 - 32) * 5/9 #?set_celsius_from_fahrenheit⋯

⋯157print("\n--- Set via Fahrenheit ---")158temp.fahrenheit→ 212.0 = 212 #?set_boiling159print(f"Celsius: {temp.celsius100.0}°C") #?print_boiling_celsius160print(f"Kelvin: {temp.kelvin}K") #?print_boiling_kelvin⋯

⋯51@property #?celsius_property52def celsius(self⟨Temperature B⟩): #?celsius_getter53    """Get temperature in Celsius."""54    return self._celsius100.0 #?return_celsius⋯

⋯158temp.fahrenheit = 212 #?set_boiling159print(f"Celsius: {temp.celsius100.0}°C") #?print_boiling_celsius160print(f"Kelvin: {temp.kelvin373.15}K") #?print_boiling_kelvin⋯

⋯73@property #?kelvin_property74def kelvin(self⟨Temperature B⟩): #?kelvin_getter75    """Computed: convert to Kelvin."""76    return self._celsius100.0 + 273.15 #?calc_kelvin⋯

⋯159print(f"Celsius: {temp.celsius}°C") #?print_boiling_celsius160print(f"Kelvin: {temp.kelvin373.15}K") #?print_boiling_kelvin161162# Rectangle computed properties #?rectangle_demo163print("\n--- Rectangle ---")164rect = Rectangle(10, 5) #?create_rect165print(f"Dimensions: {rect.width} x {rect.height}") #?print_dimensions⋯

⋯87def __init__(self⟨Rectangle C⟩, width10, height5): #?init_rectangle88    self._width→ 10 = width10 #?store_width89    self._height→ 5 = height5 #?store_height⋯

⋯163print("\n--- Rectangle ---")164rect→ ⟨Rectangle C⟩ = Rectangle(10, 5) #?create_rect165print(f"Dimensions: {rect.width10} x {rect.height5}") #?print_dimensions166print(f"Area: {rect.area}") #?print_rect_area⋯

⋯91@property #?width_property92def width(self⟨Rectangle C⟩): #?width_getter93    return self._width10 #?return_width⋯

⋯101@property #?height_property102def height(self⟨Rectangle C⟩): #?height_getter103    return self._height5 #?return_height⋯

⋯164rect = Rectangle(10, 5) #?create_rect165print(f"Dimensions: {rect.width10} x {rect.height5}") #?print_dimensions166print(f"Area: {rect.area50}") #?print_rect_area167print(f"Perimeter: {rect.perimeter}") #?print_perimeter⋯

⋯111@property #?area_rect_property112def area(self⟨Rectangle C⟩): #?area_rect_getter113    """Computed: area = width * height."""114    return self._width10 * self._height5 #?calc_rect_area⋯

⋯165print(f"Dimensions: {rect.width} x {rect.height}") #?print_dimensions166print(f"Area: {rect.area50}") #?print_rect_area167print(f"Perimeter: {rect.perimeter30}") #?print_perimeter168print(f"Aspect ratio: {rect.aspect_ratio}") #?print_aspect_ratio⋯

⋯116@property #?perimeter_property117def perimeter(self⟨Rectangle C⟩): #?perimeter_getter118    """Computed: perimeter = 2 * (width + height)."""119    return 2 * (self._width10 + self._height5) #?calc_perimeter⋯

⋯166print(f"Area: {rect.area}") #?print_rect_area167print(f"Perimeter: {rect.perimeter30}") #?print_perimeter168print(f"Aspect ratio: {rect.aspect_ratio2.0}") #?print_aspect_ratio169print(f"Is square: {rect.is_square}") #?print_is_square⋯

⋯121@property #?aspect_ratio_property122def aspect_ratio(self⟨Rectangle C⟩): #?aspect_ratio_getter123    """Computed: aspect ratio = width / height."""124    return self._width10 / self._height5 #?calc_aspect_ratio⋯

⋯167print(f"Perimeter: {rect.perimeter}") #?print_perimeter168print(f"Aspect ratio: {rect.aspect_ratio2.0}") #?print_aspect_ratio169print(f"Is square: {rect.is_squareFalse}") #?print_is_square⋯

⋯126@property #?is_square_property127def is_square(self⟨Rectangle C⟩): #?is_square_getter128    """Computed: check if it's a square."""129    return self._width10 == self._height5 #?check_square⋯

⋯168print(f"Aspect ratio: {rect.aspect_ratio}") #?print_aspect_ratio169print(f"Is square: {rect.is_squareFalse}") #?print_is_square170171# Make it a square #?make_square172print("\n--- Make it a Square ---")173rect.width = 5 #?set_width_5174print(f"Dimensions: {rect.width} x {rect.height}") #?print_square_dimensions⋯

⋯95@width.setter #?width_setter96def width(self⟨Rectangle C⟩, value5): #?width_setter_method97    if value <= 0: #?check_width98        raise ValueError("Width must be positive") #?raise_width99    self._width→ 5 = value5 #?store_valid_width⋯

⋯172print("\n--- Make it a Square ---")173rect.width→ 5 = 5 #?set_width_5174print(f"Dimensions: {rect.width5} x {rect.height5}") #?print_square_dimensions175print(f"Is square: {rect.is_square}") #?print_is_now_square⋯

⋯91@property #?width_property92def width(self⟨Rectangle C⟩): #?width_getter93    return self._width5 #?return_width⋯

⋯101@property #?height_property102def height(self⟨Rectangle C⟩): #?height_getter103    return self._height5 #?return_height⋯

⋯173rect.width = 5 #?set_width_5174print(f"Dimensions: {rect.width5} x {rect.height5}") #?print_square_dimensions175print(f"Is square: {rect.is_squareTrue}") #?print_is_now_square⋯

⋯126@property #?is_square_property127def is_square(self⟨Rectangle C⟩): #?is_square_getter128    """Computed: check if it's a square."""129    return self._width5 == self._height5 #?check_square⋯

⋯174    print(f"Dimensions: {rect.width} x {rect.height}") #?print_square_dimensions175    print(f"Is square: {rect.is_squareTrue}") #?print_is_now_square176177    print("\n=== Key Points ===")178    print("""1791. Computed properties calculate values on-the-fly1802. No need to store derived values1813. Always consistent with source data1824. Can have setters that update source data1835. Useful for conversions, calculations, flags184    """)⋯

⋯187if __name__ == "__main__":188    main()⋯

⋯6class Person:7    """Person with read-only birth_year."""89    def __init__(self, name, birth_year): #?init_person10        self._name = name #?store_name11        self._birth_year = birth_year #?store_birth_year1213    @property #?birth_year_property14    def birth_year(self): #?birth_year_getter15        """Read-only birth year."""16        return self._birth_year #?return_birth_year1718    # No @birth_year.setter - read-only! #?no_setter1920    @property #?age_property21    def age(self): #?age_getter22        """Computed age based on birth year."""23        current_year = datetime.now().year #?get_current_year24        return current_year - self._birth_year #?calc_age252627class ImmutablePoint:28    """Point with read-only coordinates."""2930    def __init__(self, x, y): #?init_point31        self._x = x #?store_x32        self._y = y #?store_y3334    @property #?x_property35    def x(self): #?x_getter36        """Read-only x coordinate."""37        return self._x #?return_x3839    @property #?y_property40    def y(self): #?y_getter41        """Read-only y coordinate."""42        return self._y #?return_y4344    @property #?distance_property45    def distance_from_origin(self): #?distance_getter46        """Computed distance from origin."""47        return (self._x ** 2 + self._y ** 2) ** 0.5 #?calc_distance484950class BankAccount:51    """Bank account with read-only account number."""5253    def __init__(self, account_number, balance=0): #?init_account54        self._account_number = account_number #?store_account_number55        self._balance = balance #?store_balance56        self._transactions = [] #?init_transactions5758    @property #?account_number_property59    def account_number(self): #?account_number_getter60        """Read-only account number."""61        return self._account_number #?return_account_number6263    @property #?balance_property64    def balance(self): #?balance_getter65        """Read-only balance (use deposit/withdraw)."""66        return self._balance #?return_balance6768    @property #?transactions_property69    def transactions(self): #?transactions_getter70        """Read-only copy of transactions."""71        return self._transactions.copy()  # Return copy, not original!7273    def deposit(self, amount): #?deposit_method74        """Deposit money."""75        if amount <= 0: #?check_deposit_amount76            raise ValueError("Deposit must be positive") #?raise_deposit77        self._balance += amount #?add_to_balance78        self._transactions.append(f"Deposit: +${amount}") #?log_deposit7980    def withdraw(self, amount): #?withdraw_method81        """Withdraw money."""82        if amount <= 0: #?check_withdraw_amount83            raise ValueError("Withdrawal must be positive") #?raise_withdraw_amount84        if amount > self._balance: #?check_sufficient_funds85            raise ValueError("Insufficient funds") #?raise_insufficient86        self._balance -= amount #?subtract_from_balance87        self._transactions.append(f"Withdrawal: -${amount}") #?log_withdrawal888990class Counter:91    """Counter with read-only count."""⋯

⋯110def main():111    print("=== Read-only Properties ===\n")112113    # Person with read-only birth year #?person_demo114    print("--- Person with Read-only Birth Year ---")115    person = Person("Alice", 1990) #?create_person116    print(f"Name: {person._name}") #?print_name⋯

⋯9def __init__(self⟨Person A⟩, nameAlice, birth_year1990): #?init_person10    self._name→ Alice = nameAlice #?store_name11    self._birth_year→ 1990 = birth_year1990 #?store_birth_year⋯

⋯114print("--- Person with Read-only Birth Year ---")115person→ ⟨Person A⟩ = Person("Alice", 1990) #?create_person116print(f"Name: {person._nameAlice}") #?print_name117print(f"Birth year: {person.birth_year1990}") #?print_birth_year118print(f"Age: {person.age}") #?print_age⋯

⋯13@property #?birth_year_property14def birth_year(self⟨Person A⟩): #?birth_year_getter15    """Read-only birth year."""16    return self._birth_year1990 #?return_birth_year⋯

⋯116print(f"Name: {person._name}") #?print_name117print(f"Birth year: {person.birth_year1990}") #?print_birth_year118print(f"Age: {person.age36}") #?print_age⋯

⋯20@property #?age_property21def age(self⟨Person A⟩): #?age_getter22    """Computed age based on birth year."""23    current_year→ 2026 = datetime<class 'datetime.datetime'>.now().year #?get_current_year24    return current_year2026 - self._birth_year1990 #?calc_age⋯

⋯117print(f"Birth year: {person.birth_year}") #?print_birth_year118print(f"Age: {person.age36}") #?print_age⋯

⋯121    person.birth_year = 1995  # Attempt to modify122except AttributeError as e: #?catch_attribute_error123    print(f"Error: {eproperty 'birth_year' of 'Person' object has no setter}") #?print_readonly_error124125# Immutable point #?point_demo⋯

⋯125# Immutable point #?point_demo126print("\n--- Immutable Point ---")127point = ImmutablePoint(3, 4) #?create_point128print(f"Point: ({point.x}, {point.y})") #?print_point⋯

⋯30def __init__(self⟨ImmutablePoint B⟩, x3, y4): #?init_point31    self._x→ 3 = x3 #?store_x32    self._y→ 4 = y4 #?store_y⋯

⋯126print("\n--- Immutable Point ---")127point→ ⟨ImmutablePoint B⟩ = ImmutablePoint(3, 4) #?create_point128print(f"Point: ({point.x3}, {point.y4})") #?print_point129print(f"Distance from origin: {point.distance_from_origin}") #?print_distance⋯

⋯34@property #?x_property35def x(self⟨ImmutablePoint B⟩): #?x_getter36    """Read-only x coordinate."""37    return self._x3 #?return_x⋯

⋯39@property #?y_property40def y(self⟨ImmutablePoint B⟩): #?y_getter41    """Read-only y coordinate."""42    return self._y4 #?return_y⋯

⋯127point = ImmutablePoint(3, 4) #?create_point128print(f"Point: ({point.x3}, {point.y4})") #?print_point129print(f"Distance from origin: {point.distance_from_origin5.0}") #?print_distance⋯

⋯44@property #?distance_property45def distance_from_origin(self⟨ImmutablePoint B⟩): #?distance_getter46    """Computed distance from origin."""47    return (self._x3 ** 2 + self._y4 ** 2) ** 0.5 #?calc_distance⋯

⋯128print(f"Point: ({point.x}, {point.y})") #?print_point129print(f"Distance from origin: {point.distance_from_origin5.0}") #?print_distance⋯

⋯132    point.x = 10 #?attempt_modify_x133except AttributeError as e: #?catch_point_error134    print(f"Error: {eproperty 'x' of 'ImmutablePoint' object has no setter}") #?print_point_error135136# Bank account #?account_demo⋯

⋯136# Bank account #?account_demo137print("\n--- Bank Account ---")138account = BankAccount("12345", 1000) #?create_account139print(f"Account: {account.account_number}") #?print_account_number⋯

⋯53def __init__(self⟨BankAccount C⟩, account_number12345, balance1000=0): #?init_account54    self._account_number→ 12345 = account_number12345 #?store_account_number55    self._balance→ 1000 = balance1000 #?store_balance56    self._transactions→ [] = [] #?init_transactions⋯

⋯137print("\n--- Bank Account ---")138account→ ⟨BankAccount C⟩ = BankAccount("12345", 1000) #?create_account139print(f"Account: {account.account_number12345}") #?print_account_number140print(f"Balance: ${account.balance}") #?print_balance⋯

⋯58@property #?account_number_property59def account_number(self⟨BankAccount C⟩): #?account_number_getter60    """Read-only account number."""61    return self._account_number12345 #?return_account_number⋯

⋯138account = BankAccount("12345", 1000) #?create_account139print(f"Account: {account.account_number12345}") #?print_account_number140print(f"Balance: ${account.balance1000}") #?print_balance⋯

⋯63@property #?balance_property64def balance(self⟨BankAccount C⟩): #?balance_getter65    """Read-only balance (use deposit/withdraw)."""66    return self._balance1000 #?return_balance⋯

⋯139print(f"Account: {account.account_number}") #?print_account_number140print(f"Balance: ${account.balance1000}") #?print_balance141142account⟨BankAccount C⟩.deposit(500) #?do_deposit143account.withdraw(200) #?do_withdraw⋯

⋯73def deposit(self⟨BankAccount C⟩, amount500): #?deposit_method74    """Deposit money."""75    if amount <= 0: #?check_deposit_amount76        raise ValueError("Deposit must be positive") #?raise_deposit77    self._balance→ 1500 += amount500 #?add_to_balance78    self._transactions→ ['Deposit: +$500'].append(f"Deposit: +${amount500}") #?log_deposit⋯

⋯142account⟨BankAccount C⟩.deposit(500) #?do_deposit143account⟨BankAccount C⟩.withdraw(200) #?do_withdraw144print(f"New balance: ${account.balance}") #?print_new_balance⋯

⋯80def withdraw(self⟨BankAccount C⟩, amount200): #?withdraw_method81    """Withdraw money."""82    if amount <= 0: #?check_withdraw_amount83        raise ValueError("Withdrawal must be positive") #?raise_withdraw_amount84    if amount > self._balance: #?check_sufficient_funds85        raise ValueError("Insufficient funds") #?raise_insufficient86    self._balance→ 1300 -= amount200 #?subtract_from_balance87    self._transactions→ ['Deposit: +$500', 'Withdrawal: -$200'].append(f"Withdrawal: -${amount200}") #?log_withdrawal⋯

⋯142account.deposit(500) #?do_deposit143account⟨BankAccount C⟩.withdraw(200) #?do_withdraw144print(f"New balance: ${account.balance1300}") #?print_new_balance⋯

⋯63@property #?balance_property64def balance(self⟨BankAccount C⟩): #?balance_getter65    """Read-only balance (use deposit/withdraw)."""66    return self._balance1300 #?return_balance⋯

⋯143account.withdraw(200) #?do_withdraw144print(f"New balance: ${account.balance1300}") #?print_new_balance145146print("Transactions:") #?print_transactions_header147for transaction in account.transactions: #?loop_transactions⋯

⋯68@property #?transactions_property69def transactions(self⟨BankAccount C⟩): #?transactions_getter70    """Read-only copy of transactions."""71    return self._transactions['Deposit: +$500', 'Withdrawal: -$200'].copy()  # Return copy, not original!⋯

⋯146print("Transactions:") #?print_transactions_header147for transactionDeposit: +$500 in account.transactions['Deposit: +$500', 'Withdrawal: -$200']: #?loop_transactions148    print(f"  - {transactionDeposit: +$500}") #?print_transaction⋯

⋯146print("Transactions:") #?print_transactions_header147for transactionWithdrawal: -$200 in account.transactions['Deposit: +$500', 'Withdrawal: -$200']: #?loop_transactions148    print(f"  - {transactionWithdrawal: -$200}") #?print_transaction⋯

⋯151    account.balance = 999999  # Attempt to cheat152except AttributeError as e: #?catch_balance_error153    print(f"Error: {eproperty 'balance' of 'BankAccount' object has no setter}") #?print_balance_error154155# Counter #?counter_demo⋯

⋯155# Counter #?counter_demo156print("\n--- Counter ---")157counter = Counter() #?create_counter158print(f"Initial count: {counter.count}") #?print_initial_count⋯

⋯93def __init__(self⟨Counter D⟩): #?init_counter94    self._count→ 0 = 0 #?init_count⋯

⋯156print("\n--- Counter ---")157counter→ ⟨Counter D⟩ = Counter() #?create_counter158print(f"Initial count: {counter.count0}") #?print_initial_count⋯

⋯96@property #?count_property97def count(self⟨Counter D⟩): #?count_getter98    """Read-only count."""99    return self._count0 #?return_count⋯

⋯157counter = Counter() #?create_counter158print(f"Initial count: {counter.count0}") #?print_initial_count159160counter⟨Counter D⟩.increment() #?inc1161counter.increment() #?inc2⋯

⋯101def increment(self⟨Counter D⟩): #?increment_method102    """Increment counter."""103    self._count→ 1 += 1 #?do_increment⋯

⋯160counter⟨Counter D⟩.increment() #?inc1161counter⟨Counter D⟩.increment() #?inc2162counter.increment() #?inc3⋯

⋯160counter.increment() #?inc1161counter⟨Counter D⟩.increment() #?inc2162counter⟨Counter D⟩.increment() #?inc3163print(f"After increments: {counter.count}") #?print_after_increments⋯

⋯161counter.increment() #?inc2162counter⟨Counter D⟩.increment() #?inc3163print(f"After increments: {counter.count3}") #?print_after_increments⋯

⋯162counter.increment() #?inc3163print(f"After increments: {counter.count3}") #?print_after_increments⋯

⋯166    counter.count = 100 #?attempt_modify_count167except AttributeError as e: #?catch_count_error168    print(f"Error: {eproperty 'count' of 'Counter' object has no setter}") #?print_count_error169170counter.reset() #?do_reset⋯

⋯170counter⟨Counter D⟩.reset() #?do_reset171print(f"After reset: {counter.count}") #?print_after_reset⋯

⋯105def reset(self⟨Counter D⟩): #?reset_method106    """Reset counter."""107    self._count→ 0 = 0 #?do_reset⋯

⋯170counter⟨Counter D⟩.reset() #?do_reset171print(f"After reset: {counter.count0}") #?print_after_reset⋯

⋯170    counter.reset() #?do_reset171    print(f"After reset: {counter.count0}") #?print_after_reset172173    print("\n=== Key Points ===")174    print("""1751. Property without setter is read-only1762. Attempting to set raises AttributeError1773. Useful for IDs, computed values, immutable data1784. Provide methods (deposit/withdraw) instead of direct access1795. Return copies of mutable internal data180    """)⋯

⋯183if __name__ == "__main__":184    main()⋯

⋯4class UserV1:5    """Original version with direct attribute access."""67    def __init__(self, username, email): #?init_v18        self.username = username  # Direct attribute9        self.email = email  # Direct attribute101112# Version 2: Using properties (new way - backward compatible!) #?version213class UserV2:14    """Refactored with properties - same interface!"""1516    def __init__(self, username, email): #?init_v217        self.username = username  # Uses setter18        self.email = email  # Uses setter1920    @property #?username_property_v221    def username(self): #?username_getter_v222        """Get username."""23        return self._username #?return_username_v22425    @username.setter #?username_setter_v226    def username(self, value): #?username_setter_method_v227        """Set username with validation (NEW!)."""28        if not isinstance(value, str) or len(value) < 3: #?validate_username_v229            raise ValueError("Username must be string with 3+ chars") #?raise_username_v230        self._username = value #?store_username_v23132    @property #?email_property_v233    def email(self): #?email_getter_v234        """Get email."""35        return self._email #?return_email_v23637    @email.setter #?email_setter_v238    def email(self, value): #?email_setter_method_v239        """Set email with validation (NEW!)."""40        if "@" not in value: #?validate_email_v241            raise ValueError("Invalid email format") #?raise_email_v242        self._email = value.lower()  # Normalize to lowercase (NEW!)434445# Old API - direct access #?old_api46class LegacyCart:47    """Old shopping cart with direct access."""4849    def __init__(self): #?init_legacy_cart50        self.items = [] #?direct_items51        self.total = 0 #?direct_total5253    def add_item(self, price): #?add_item_legacy54        """Add item (user must update total manually - error-prone!)."""55        self.items.append(price) #?append_item56        # User responsible for updating total! #?manual_total575859# New API - using properties #?new_api60class ModernCart:61    """Modernized cart with computed total."""⋯

⋯84def main():85    print("=== Migrating to Properties ===\n")8687    # Version 1: Direct attribute access #?demo_v188    print("--- Version 1: Direct Attributes ---")89    user_v1 = UserV1("alice", "ALICE@EXAMPLE.COM") #?create_v190    print(f"Username: {user_v1.username}") #?print_v1_username⋯

⋯7def __init__(self⟨UserV1 A⟩, usernamealice, emailALICE@EXAMPLE.COM): #?init_v18    self.username→ alice = usernamealice  # Direct attribute9    self.email→ ALICE@EXAMPLE.COM = emailALICE@EXAMPLE.COM  # Direct attribute⋯

⋯88print("--- Version 1: Direct Attributes ---")89user_v1→ ⟨UserV1 A⟩ = UserV1("alice", "ALICE@EXAMPLE.COM") #?create_v190print(f"Username: {user_v1.usernamealice}") #?print_v1_username91print(f"Email: {user_v1.emailALICE@EXAMPLE.COM}") #?print_v1_email9293# Problems with V1: #?v1_problems94user_v1.username→ ab = "ab"  # No validation - BAD DATA!95user_v1.email→ not-an-email = "not-an-email"  # No validation - BAD DATA!96print(f"Invalid username accepted: {user_v1.usernameab}") #?print_invalid_v197print(f"Invalid email accepted: {user_v1.emailnot-an-email}") #?print_invalid_email_v19899# Version 2: Properties (backward compatible interface!) #?demo_v2100print("\n--- Version 2: Properties ---")101user_v2 = UserV2("alice", "ALICE@EXAMPLE.COM") #?create_v2102print(f"Username: {user_v2.username}")  # Same interface!⋯

⋯16def __init__(self⟨UserV2 B⟩, usernamealice, emailALICE@EXAMPLE.COM): #?init_v217    self.username = usernamealice  # Uses setter18    self.email = email  # Uses setter⋯

⋯16def __init__(self, username, email): #?init_v217    self.username→ alice = usernamealice  # Uses setter18    self.email = emailALICE@EXAMPLE.COM  # Uses setter1920@property #?username_property_v221def username(self): #?username_getter_v222    """Get username."""23    return self._username #?return_username_v22425@username.setter #?username_setter_v226def username(self⟨UserV2 B⟩, valuealice): #?username_setter_method_v227    """Set username with validation (NEW!)."""28    if not isinstance(value, str) or len(value) < 3: #?validate_username_v229        raise ValueError("Username must be string with 3+ chars") #?raise_username_v230    self._username→ alice = valuealice #?store_username_v2⋯

⋯17    self.username = username  # Uses setter18    self.email→ alice@example.com = emailALICE@EXAMPLE.COM  # Uses setter1920@property #?username_property_v221def username(self): #?username_getter_v222    """Get username."""23    return self._username #?return_username_v22425@username.setter #?username_setter_v226def username(self, value): #?username_setter_method_v227    """Set username with validation (NEW!)."""28    if not isinstance(value, str) or len(value) < 3: #?validate_username_v229        raise ValueError("Username must be string with 3+ chars") #?raise_username_v230    self._username = value #?store_username_v23132@property #?email_property_v233def email(self): #?email_getter_v234    """Get email."""35    return self._email #?return_email_v23637@email.setter #?email_setter_v238def email(self⟨UserV2 B⟩, valueALICE@EXAMPLE.COM): #?email_setter_method_v239    """Set email with validation (NEW!)."""40    if "@" not in value: #?validate_email_v241        raise ValueError("Invalid email format") #?raise_email_v242    self._email→ alice@example.com = valueALICE@EXAMPLE.COM.lower()  # Normalize to lowercase (NEW!)⋯

⋯100print("\n--- Version 2: Properties ---")101user_v2→ ⟨UserV2 B⟩ = UserV2("alice", "ALICE@EXAMPLE.COM") #?create_v2102print(f"Username: {user_v2.usernamealice}")  # Same interface!103print(f"Email: {user_v2.email}")  # But normalized!⋯

⋯20@property #?username_property_v221def username(self⟨UserV2 B⟩): #?username_getter_v222    """Get username."""23    return self._usernamealice #?return_username_v2⋯

⋯101user_v2 = UserV2("alice", "ALICE@EXAMPLE.COM") #?create_v2102print(f"Username: {user_v2.usernamealice}")  # Same interface!103print(f"Email: {user_v2.emailalice@example.com}")  # But normalized!⋯

⋯32@property #?email_property_v233def email(self⟨UserV2 B⟩): #?email_getter_v234    """Get email."""35    return self._emailalice@example.com #?return_email_v2⋯

⋯102print(f"Username: {user_v2.username}")  # Same interface!103print(f"Email: {user_v2.emailalice@example.com}")  # But normalized!⋯

⋯25@username.setter #?username_setter_v226def username(self⟨UserV2 B⟩, valueab): #?username_setter_method_v227    """Set username with validation (NEW!)."""28    if not isinstance(value, str) or len(value) < 3: #?validate_username_v2⋯

⋯27"""Set username with validation (NEW!)."""28if not isinstance(valueab, str) or len(value) < 3: #?validate_username_v229    raise ValueError("Username must be string with 3+ chars") #?raise_username_v230self._username = value #?store_username_v2⋯

⋯107    user_v2.username = "ab"  # Now validated!108except ValueError as e: #?catch_v2_error109    print(f"Validation error: {eUsername must be string with 3+ chars}") #?print_v2_error110111# Legacy cart problems #?legacy_cart_demo⋯

⋯111# Legacy cart problems #?legacy_cart_demo112print("\n--- Legacy Cart (Manual Total) ---")113legacy = LegacyCart() #?create_legacy114legacy.add_item(10.00) #?add_legacy_1⋯

⋯49def __init__(self⟨LegacyCart C⟩): #?init_legacy_cart50    self.items→ [] = [] #?direct_items51    self.total→ 0 = 0 #?direct_total⋯

⋯112print("\n--- Legacy Cart (Manual Total) ---")113legacy→ ⟨LegacyCart C⟩ = LegacyCart() #?create_legacy114legacy⟨LegacyCart C⟩.add_item(10.00) #?add_legacy_1115legacy.add_item(20.00) #?add_legacy_2⋯

⋯53def add_item(self⟨LegacyCart C⟩, price10.0): #?add_item_legacy54    """Add item (user must update total manually - error-prone!)."""55    self.items→ [10.0].append(price10.0) #?append_item56    # User responsible for updating total! #?manual_total⋯

⋯113legacy = LegacyCart() #?create_legacy114legacy⟨LegacyCart C⟩.add_item(10.00) #?add_legacy_1115legacy⟨LegacyCart C⟩.add_item(20.00) #?add_legacy_2116print(f"Items: {legacy.items}") #?print_legacy_items⋯

⋯53def add_item(self⟨LegacyCart C⟩, price20.0): #?add_item_legacy54    """Add item (user must update total manually - error-prone!)."""55    self.items→ [10.0, 20.0].append(price20.0) #?append_item56    # User responsible for updating total! #?manual_total⋯

⋯114legacy.add_item(10.00) #?add_legacy_1115legacy⟨LegacyCart C⟩.add_item(20.00) #?add_legacy_2116print(f"Items: {legacy.items[10.0, 20.0]}") #?print_legacy_items117print(f"Total: ${legacy.total0}")  # Still 0 - forgot to update!118119# Forgot to update total! #?forgot_total120legacy.total→ 30.0 = 30.00  # Manual and error-prone121print(f"After manual update: ${legacy.total30.0}") #?print_manual_total122123# Modern cart with computed property #?modern_cart_demo124print("\n--- Modern Cart (Computed Total) ---")125modern = ModernCart() #?create_modern126modern.add_item(10.00) #?add_modern_1⋯

⋯63def __init__(self⟨ModernCart D⟩): #?init_modern_cart64    self._items→ [] = [] #?private_items⋯

⋯124print("\n--- Modern Cart (Computed Total) ---")125modern→ ⟨ModernCart D⟩ = ModernCart() #?create_modern126modern⟨ModernCart D⟩.add_item(10.00) #?add_modern_1127modern.add_item(20.00) #?add_modern_2⋯

⋯76def add_item(self⟨ModernCart D⟩, price10.0): #?add_item_modern77    """Add item (total auto-updates!)."""78    if price < 0: #?validate_price79        raise ValueError("Price cannot be negative") #?raise_price80    self._items→ [10.0].append(price10.0) #?append_modern_item81    # No manual total update needed! #?auto_total⋯

⋯125modern = ModernCart() #?create_modern126modern⟨ModernCart D⟩.add_item(10.00) #?add_modern_1127modern⟨ModernCart D⟩.add_item(20.00) #?add_modern_2128print(f"Items: {modern.items}") #?print_modern_items⋯

⋯126modern.add_item(10.00) #?add_modern_1127modern⟨ModernCart D⟩.add_item(20.00) #?add_modern_2128print(f"Items: {modern.items[10.0, 20.0]}") #?print_modern_items129print(f"Total: ${modern.total}")  # Always correct!⋯

⋯66@property #?items_property67def items(self⟨ModernCart D⟩): #?items_getter68    """Get items (read-only copy)."""69    return self._items[10.0, 20.0].copy() #?return_items_copy⋯

⋯127modern.add_item(20.00) #?add_modern_2128print(f"Items: {modern.items[10.0, 20.0]}") #?print_modern_items129print(f"Total: ${modern.total30.0}")  # Always correct!⋯

⋯71@property #?total_property72def total(self⟨ModernCart D⟩): #?total_getter73    """Computed total (always accurate!)."""74    return sum(self._items[10.0, 20.0]) #?calc_total⋯

⋯128print(f"Items: {modern.items}") #?print_modern_items129print(f"Total: ${modern.total30.0}")  # Always correct!130131modern⟨ModernCart D⟩.add_item(15.00) #?add_modern_3132print(f"After adding item: ${modern.total}")  # Auto-updated!⋯

⋯131modern⟨ModernCart D⟩.add_item(15.00) #?add_modern_3132print(f"After adding item: ${modern.total45.0}")  # Auto-updated!⋯

⋯71@property #?total_property72def total(self⟨ModernCart D⟩): #?total_getter73    """Computed total (always accurate!)."""74    return sum(self._items[10.0, 20.0, 15.0]) #?calc_total⋯

⋯131modern.add_item(15.00) #?add_modern_3132print(f"After adding item: ${modern.total45.0}")  # Auto-updated!⋯

⋯136    modern.total = 9999 #?attempt_set_total137except AttributeError as e: #?catch_total_error138    print(f"Error: {eproperty 'total' of 'ModernCart' object has no setter}") #?print_total_error139140print("\n=== Migration Benefits ===")⋯

⋯140    print("\n=== Migration Benefits ===")141    print("""1421. Backward compatible - same interface1432. Add validation without breaking existing code1443. Transform data (e.g., normalize email)1454. Computed values always accurate1465. Prevent incorrect manual updates1476. Read-only computed properties148    """)⋯

⋯151if __name__ == "__main__":152    main()⋯