4. Descriptor Protocols

These examples demonstrate how Python's descriptor protocol (__get__, __set__, and __set_name__) allows fine-grained control over attribute access and modification.

1. Basic Descriptor Example

Copy

class Descriptor:
    def __get__(self, instance, owner):
        print("Getting value")
        return instance._value
    
    def __set__(self, instance, value):
        print("Setting value")
        instance._value = value

class MyClass:
    attribute = Descriptor()

obj = MyClass()
obj.attribute = 42  # Setting value
print(obj.attribute)  # Getting value, Output: 42

---

2. Read-Only Descriptor

Copy

class ReadOnlyDescriptor:
    def __get__(self, instance, owner):
        return "This is a read-only attribute"

    def __set__(self, instance, value):
        raise AttributeError("Cannot modify read-only attribute")

class MyClass:
    read_only = ReadOnlyDescriptor()

obj = MyClass()
print(obj.read_only)  # Output: This is a read-only attribute
# obj.read_only = "New value"  # Raises AttributeError

---

3. Descriptor with Validation

Copy

class PositiveNumber:
    def __get__(self, instance, owner):
        return instance._number

    def __set__(self, instance, value):
        if value < 0:
            raise ValueError("Number must be positive")
        instance._number = value

class MyClass:
    number = PositiveNumber()

obj = MyClass()
obj.number = 10
print(obj.number)  # Output: 10
# obj.number = -5  # Raises ValueError

---

4. Non-Data Descriptor

Copy

class NonDataDescriptor:
    def __get__(self, instance, owner):
        return "Non-data descriptor called"

class MyClass:
    attribute = NonDataDescriptor()

obj = MyClass()
print(obj.attribute)  # Output: Non-data descriptor called

---

5. Descriptor in Class Variables

Copy

class Descriptor:
    def __get__(self, instance, owner):
        return "Class-level descriptor"

class MyClass:
    attribute = Descriptor()

print(MyClass.attribute)  # Output: Class-level descriptor

---

6. Combining __get__ and __set__

Copy

class Descriptor:
    def __get__(self, instance, owner):
        return instance.__dict__.get("_value", None)

    def __set__(self, instance, value):
        print(f"Setting value to {value}")
        instance.__dict__["_value"] = value

class MyClass:
    attribute = Descriptor()

obj = MyClass()
obj.attribute = "Hello"
print(obj.attribute)  # Output: Hello

---

7. Tracking Access with a Descriptor

Copy

class TrackingDescriptor:
    def __init__(self):
        self.access_count = 0

    def __get__(self, instance, owner):
        self.access_count += 1
        return f"Accessed {self.access_count} times"

class MyClass:
    tracked = TrackingDescriptor()

obj = MyClass()
print(obj.tracked)  # Output: Accessed 1 times
print(obj.tracked)  # Output: Accessed 2 times

---

8. Descriptor to Control Multiple Attributes

Copy

class ControlledAttribute:
    def __get__(self, instance, owner):
        return instance.__dict__.get(self.name)

    def __set__(self, instance, value):
        print(f"Setting {self.name} to {value}")
        instance.__dict__[self.name] = value

    def __set_name__(self, owner, name):
        self.name = name

class MyClass:
    attr1 = ControlledAttribute()
    attr2 = ControlledAttribute()

obj = MyClass()
obj.attr1 = "Value 1"
obj.attr2 = "Value 2"
print(obj.attr1, obj.attr2)  # Output: Value 1 Value 2

---

9. Using a Descriptor for Logging

Copy

class LoggedDescriptor:
    def __get__(self, instance, owner):
        print(f"Accessing {self.name}")
        return instance.__dict__.get(self.name)

    def __set__(self, instance, value):
        print(f"Setting {self.name} to {value}")
        instance.__dict__[self.name] = value

    def __set_name__(self, owner, name):
        self.name = name

class MyClass:
    logged_attr = LoggedDescriptor()

obj = MyClass()
obj.logged_attr = "Testing"
print(obj.logged_attr)

---

10. Descriptor with Property-Like Behavior

Copy

class Temperature:
    def __get__(self, instance, owner):
        return (instance._celsius * 9/5) + 32  # Convert Celsius to Fahrenheit

    def __set__(self, instance, value):
        instance._celsius = value  # Store value in Celsius

class Weather:
    temperature = Temperature()

weather = Weather()
weather.temperature = 25  # Setting in Celsius
print(weather.temperature)  # Output: 77.0 (Fahrenheit)

---