192. Descriptor Chaining

1. Basic Descriptor

• A simple descriptor controlling attribute access.

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class UpperCaseDescriptor:
    def __get__(self, instance, owner):
        return instance.__dict__.get("_name", "").upper()
    
    def __set__(self, instance, value):
        instance.__dict__["_name"] = value

class Person:
    name = UpperCaseDescriptor()  # Using the descriptor

p = Person()
p.name = "alice"
print(p.name)  # Output: ALICE

🔍 How it works:

• The descriptor automatically converts name to uppercase when accessed.

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2. Chaining Two Descriptors

• Combining two descriptors to control access.

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class UpperCaseDescriptor:
    def __get__(self, instance, owner):
        return instance.__dict__.get("_name", "").upper()
    
    def __set__(self, instance, value):
        instance.__dict__["_name"] = value

class LengthLimitDescriptor:
    def __init__(self, limit):
        self.limit = limit

    def __get__(self, instance, owner):
        return instance.__dict__.get("_name", "")

    def __set__(self, instance, value):
        if len(value) > self.limit:
            raise ValueError(f"Name must be at most {self.limit} characters long.")
        instance.__dict__["_name"] = value

class Person:
    name = LengthLimitDescriptor(10)
    name = UpperCaseDescriptor()  # Chaining descriptors

p = Person()
p.name = "short"
print(p.name)  # Output: SHORT

🔍 How it works:

• The last assigned descriptor (UpperCaseDescriptor) takes precedence.

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3. Combining Read-Only and Validation Descriptors

• Using two descriptors: one for validation and one for read-only access.

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class ReadOnly:
    def __get__(self, instance, owner):
        return instance.__dict__["_name"]
    
    def __set__(self, instance, value):
        raise AttributeError("Cannot modify this attribute.")

class ValidateLength:
    def __init__(self, limit):
        self.limit = limit

    def __set__(self, instance, value):
        if len(value) > self.limit:
            raise ValueError(f"Name cannot exceed {self.limit} characters.")
        instance.__dict__["_name"] = value

class Person:
    name = ValidateLength(8)
    name = ReadOnly()  # Chaining

p = Person()
p.name = "Max"
print(p.name)  # Output: Max
p.name = "TooLongName"  # Raises ValueError

🔍 How it works:

• ValidateLength first validates the input.

• ReadOnly then prevents modification.

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4. Logging Access with a Descriptor

• Logs access before returning the attribute.

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class Logger:
    def __get__(self, instance, owner):
        print("Accessing name attribute")
        return instance.__dict__.get("_name", "")

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

class Person:
    name = Logger()  # Using logging descriptor

p = Person()
p.name = "John"
print(p.name)

🔍 How it works:

• Logs every read and write operation.

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5. Using Property with Descriptors

• Combining property() with a descriptor.

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class Capitalize:
    def __get__(self, instance, owner):
        return instance.__dict__["_name"].capitalize()

    def __set__(self, instance, value):
        instance.__dict__["_name"] = value

class Person:
    _name = ""
    
    @property
    def name(self):
        return self._name
    
    @name.setter
    def name(self, value):
        self._name = Capitalize().__get__(self, Person)  # Using the descriptor

p = Person()
p.name = "john"
print(p.name)  # Output: John

🔍 How it works:

• property() is used along with the Capitalize descriptor.

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