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What does the @property decorator do, and how does it let a method be accessed like an attribute? How would you convert an existing plain attribute into a property later, without breaking any existing code that reads or writes it directly? What is a descriptor, and which dunder methods define the descriptor protocol? What is the difference between a data descriptor and a non-data descriptor, and how does this affect attribute lookup priority? How does __set_name__ let a descriptor automatically learn the attribute name it's being assigned to on the owning class? How would you implement a lazy, cached property using a custom descriptor (or functools.cached_property) that only computes its value once? How do you write a descriptor that behaves differently when accessed on the CLASS itself versus on an INSTANCE? How would you implement a type-validating descriptor that enforces a specific type constraint, reusable across multiple attributes and classes? How does Python's own implementation of instance methods use the descriptor protocol to achieve automatic 'self' binding? How do you implement a descriptor-based ORM-style field that lazily loads related data only when actually accessed?
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How do you add a setter to a property so assignment (obj.attr = value) triggers custom logic, like validation?

Beginner

Answer

Define a second method with the SAME name, decorated with @property_name.setter — this method runs whenever someone assigns to the property, letting you validate or transform the incoming value before storing it (typically in a differently-named private attribute).
class Temperature:
    def __init__(self, celsius):
        self._celsius = celsius
    @property
    def celsius(self):
        return self._celsius
    @celsius.setter
    def celsius(self, value):
        if value < -273.15:
            raise ValueError("Below absolute zero!")
        self._celsius = value

t = Temperature(20)
t.celsius = 25  # runs the setter's validation logic
Real-world example Validating that a value assigned to an attribute (like temperature or age) meets certain constraints before accepting it.

Common follow-ups: Why does the underlying stored value typically need a DIFFERENT name (like _celsius) from the property itself (celsius)?

Exception Handling
All 15 Descriptors & Properties questions