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What does the @property decorator do, and how does it let a method be accessed like an attribute? How do you add a setter to a property so assignment (obj.attr = value) triggers custom logic, like validation? 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 would you convert an existing plain attribute into a property later, without breaking any existing code that reads or writes it directly?

Beginner

Answer

Since attribute access syntax (obj.attr) looks identical whether it's a plain instance attribute or a property, you can freely replace a plain attribute with an @property (backed by a renamed private attribute) at any time without changing any external code that reads or assigns to it — this is a key reason Python developers don't need to write getters/setters defensively upfront.
# Before: plain attribute
class Temperature:
    def __init__(self, celsius):
        self.celsius = celsius

# After: converted to a property, external code (t.celsius) is unaffected
class Temperature:
    def __init__(self, celsius):
        self._celsius = celsius
    @property
    def celsius(self):
        return self._celsius
    @celsius.setter
    def celsius(self, value):
        self._celsius = value
Real-world example Adding validation to a class's attribute later in a project's life, without needing to update every place that already reads or writes that attribute.

Common follow-ups: Why is this seamless upgrade path considered a key advantage of Python's approach compared to languages that require explicit getters/setters from the start?

OOP
All 15 Descriptors & Properties questions