How are class or object properties declared in a programming language?

In Python, class or object properties are declared using variables inside the class. To create managed properties, the @property decorator is used.

Example of declaring a simple property and a property with a getter and setter:

class MyClass:
    def __init__(self, value):
        self._value = value  # private field

    @property
    def value(self):
        return self._value

    @value.setter
    def value(self, new_value):
        if new_value < 0:
            raise ValueError("Value cannot be negative")
        self._value = new_value

obj = MyClass(10)
print(obj.value)  # calls getter
obj.value = 20    # calls setter

Thus, properties allow controlling access to object data, ensuring encapsulation.