> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # Python Property Setter A property setter in Python is a method decorated with `@.setter` that intercepts and handles assignment operations to a managed attribute. It acts as the `__set__` component of Python's descriptor protocol, allowing developers to execute underlying logic when an instance's property is mutated via the assignment operator (`=`). To define a setter, a getter method must first be established using the `@property` decorator. The setter method must share the exact same name as the getter and accept exactly two parameters: the instance reference (`self`) and the new value being assigned. ```python theme={"dark"} class Entity: def __init__(self): self._managed_attribute = None # 1. Define the getter first using @property @property def attribute_name(self): return self._managed_attribute # 2. Define the setter using @.setter @attribute_name.setter def attribute_name(self, value): self._managed_attribute = value ``` ## Execution Mechanics When an assignment occurs (e.g., `instance.attribute_name = "new_data"`), Python translates this operation into a method call to the decorated setter, passing `"new_data"` as the `value` argument. If a setter is not defined for a property, attempting to assign a value to it will raise an `AttributeError: can't set attribute`. ## Descriptor Protocol Implementation Under the hood, the built-in `@property` decorator is a class that implements the descriptor protocol (`__get__`, `__set__`, `__delete__`). When the `@property` decorator is applied to the getter, it creates a property object in the class namespace. This property object possesses its own `.setter()` method. When you apply the `@attribute_name.setter` decorator, Python invokes this method on the existing property object. It returns a new property object containing both the original getter and the newly bound setter function, and reassigns it to the same name in the class dictionary. The decorator syntax is syntactic sugar for the following programmatic implementation: ```python theme={"dark"} class Entity: def __init__(self): self._managed_attribute = None def _get_attribute(self): return self._managed_attribute def _set_attribute(self, value): self._managed_attribute = value # Manually constructing the property descriptor with fget and fset attribute_name = property(fget=_get_attribute, fset=_set_attribute) ```

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