Descriptors in Python (__get__, __set__, __delete__)

Descriptors are advanced Python objects that control attribute access in Python.

They are widely used in: - Django ORM fields - Pydantic validation - Flask internals - @property system

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What is a Descriptor?

A descriptor is any object that defines at least one of:

• __get__
• __set__
• __delete__

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1. __get__ (Attribute Access)

Used when you access:

obj.value

Example:

class Descriptor:
    def __get__(self, instance, owner):
        return "Hello from descriptor"

class MyClass:
    value = Descriptor()

obj = MyClass()
print(obj.value)

Output:

Hello from descriptor

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Graph

obj.value
   ↓
__get__ is called
   ↓
value returned

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Real World Example (Logging Access)

class LogAccess:
    def __get__(self, instance, owner):
        print("Accessing attribute")
        return instance._name

class User:
    name = LogAccess()

    def __init__(self, name):
        self._name = name

u = User("Aman")
print(u.name)

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2. __set__ (Attribute Assignment)

Used when:

obj.value = something

Example:

class Validator:
    def __set__(self, instance, value):
        if value < 18:
            raise ValueError("Age must be >= 18")
        instance._age = value

class Person:
    age = Validator()

p = Person()
p.age = 20
print(p._age)

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Graph

p.age = 20
   ↓
__set__ triggered
   ↓
validation runs
   ↓
value stored

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Real World Example

Used in: - Django model fields - API validation - Forms

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3. __delete__ (Attribute Deletion)

Used when:

del obj.value

Example:

class SafeDelete:
    def __delete__(self, instance):
        print("Deleting value")
        del instance._name

class User:
    name = SafeDelete()

    def __init__(self, name):
        self._name = name

u = User("Aman")
del u.name

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Graph

del obj.name
     ↓
__delete__ runs
     ↓
cleanup executed

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4. Full Descriptor (All Methods)

class NameDescriptor:
    def __get__(self, instance, owner):
        return instance._name

    def __set__(self, instance, value):
        if not value:
            raise ValueError("Name cannot be empty")
        instance._name = value

    def __delete__(self, instance):
        print("Name deleted")
        del instance._name

class User:
    name = NameDescriptor()

    def __init__(self, name):
        self.name = name

u = User("Aman")
print(u.name)
u.name = "Ravi"
del u.name

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Descriptor Flow

Attribute Access
      ↓
Python checks descriptor
      ↓
__get__ / __set__ / __delete__
      ↓
Custom logic executes

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5. Real World Mapping

Action	Meaning
__get__	read DB field
__set__	validate input
__delete__	cleanup/security

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6. Descriptor vs @property

Feature	Descriptor	@property
Reusability	High	Low
Power	Very High	Medium
Framework usage	Heavy	Light

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7. How @property works internally

@property is actually a descriptor.

Flow: @property → __get__ internally → controlled attribute access

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8. Common Mistakes

Mistake 1: Missing parameters

def __get__(self):

Correct:

def __get__(self, instance, owner):

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Mistake 2: Wrong storage

self.age = value   ❌

Correct:

self._age = value   ✔

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Mistake 3: Using descriptor on instance

obj = Descriptor()

Correct:

class A:
    value = Descriptor()

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Homework 1

Create Age Descriptor: - age must be >= 18

class Age:
    def __get__(self, instance, owner):
        return instance._age

    def __set__(self, instance, value):
        if value < 18:
            raise ValueError("Too young")
        instance._age = value

class Person:
    age = Age()

    def __init__(self, age):
        self.age = age

p = Person(20)
print(p.age)

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Homework 2

Logging descriptor:

class Logger:
    def __get__(self, instance, owner):
        print("Accessed")
        return instance._value

    def __set__(self, instance, value):
        print("Updated")
        instance._value = value

class Data:
    value = Logger()

d = Data()
d.value = 100
print(d.value)

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9. Real World Use Cases

System	Usage
Django ORM	Field validation
Flask	Request handling
Pydantic	Data validation
Python internals	property system

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10. Visual Summary

Object attribute access ↓ Descriptor protocol triggered ↓ Custom logic executes ↓ Final value returned

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Next Advanced Topics

• Metaclasses (VERY important next step)
• Django ORM internals
• __getattribute__ deep behavior
• Pydantic validation system
• Property vs Descriptor advanced patterns