Descriptors Deep Dive

Introduction

Deep dive into descriptors for reusable property behavior with full protocol implementation.

Non-Data vs Data Descriptors

# Non-data descriptor (read-only)
class NonDataDescriptor:
    def __get__(self, obj, objtype=None):
        return "computed value"

# Data descriptor (overrides instance __dict__)
class DataDescriptor:
    def __get__(self, obj, objtype=None):
        return obj.__dict__.get("_value", "default")
    
    def __set__(self, obj, value):
        obj.__dict__["_value"] = value

Property as Data Descriptor

class Circle:
    def __init__(self, radius):
        self.radius = radius  # Triggers __set__
    
    @property
    def diameter(self):
        return self.radius * 2
    
    @diameter.setter
    def diameter(self, value):
        self.radius = value / 2

Shared Descriptor Behavior

class ValidatedProperty:
    def __init__(self, min_val=None, max_val=None):
        self.min_val = min_val
        self.max_val = max_val
    
    def __set_name__(self, owner, name):
        self.name = name
    
    def __get__(self, obj, objtype=None):
        return obj.__dict__[self.name]
    
    def __set__(self, obj, value):
        if self.min_val is not None and value < self.min_val:
            raise ValueError(f"{self.name} must be >= {self.min_val}")
        obj.__dict__[self.name] = value

Practice Problems

1. Create validated numeric descriptor
2. Implement lazy loading descriptor
3. Create read-only descriptor
4. Build descriptor with callback
5. Use descriptors in data classes