Descriptor Protocol for Type Safety

Snippet

Descriptor Protocol for Type Safety

Descriptors are classes that define any of the methods __get__, __set__, or __delete__. They are used to implement custom logic for attribute access. In this snippet, __set_name__ automatically captures the attribute name, and __set__ enforces that the value assigned to 'age' is always an integer.

snippet.py
python
class IntegerField:
    def __set_name__(self, owner, name):
        self.name = name
 
    def __set__(self, instance, value):
        if not isinstance(value, int):
            raise ValueError(f'{self.name} must be an integer')
        instance.__dict__[self.name] = value
 
class User:
    age = IntegerField()

Breakdown

def __set_name__(self, owner, name):

Called at class creation time to know the name assigned to the descriptor.

def __set__(self, instance, value):

Invoked whenever the managed attribute is assigned a value.

instance.__dict__[self.name] = value

Stores the value in the instance's dictionary to avoid infinite recursion.

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Descriptor Protocol for Type Safety

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