Meaning of Python's @setter decorator

In the website https://stackoverflow.com/questions/17330160/how-does-the-property-decorator-work-in-python you can find many answers about the meaning of the @property decorator, but no answer about the corresponding @propertyname.setter decorator.

My question is the following. In the code below, is it possible to replace the @x.setter decorator with a single statement?

class D:
    def __init__(self):
        self._x = 'Hi'

    def x(self):
        return self._x
    x = property(x)

    @x.setter #delete this decorator
    def x(self, value):
       self._x = value
    # Your code, one line!

There's no way to replace the decorator syntax for your setter method with exactly one line. That's because in this specific situation, the little detail of how applying the decorator manually differs from @decorator syntax matters.

That difference is that with @decorator syntax, the function to be decorated is never bound to a name before it's passed to the decorator. In your code, the x name is already in use in our namespace, for the property object decorating the getter function. If we temporarily bind x to the undecorated setter function, we'll lose access to that object, and we need it, since the decorator we want to apply is one of its attributes!

You can work around this by giving the setter function a different name:

class D:
    def __init__(self):
        self._x = 'Hi'

    def x(self):
        return self._x
    x = property(x)   # this is the previous x value, which we will eventually overwrite
    
    def x_setter(self, value):   # rename this function, so we don't clobber x quite yet
       self._x = value
    x = x.setter(x_setter)       # then apply the decorator manually, overwriting x

    # we might want to `del x_setter` here, to avoid cluttering the namespace

As my comments show, the previous x value does eventually get overwritten, by our new property that wraps both the getter and setter methods. But we can't do it too soon, or we won't be able to access x.setter using the old version of x.

A more natural way to call property manually with both getter and setter methods is to pass both of them (and maybe a deleter too) in a single property call:

class D:
    def __init__(self):
        self._x = 'Hi'

    def x_getter(self):
        return self._x

    def x_setter(self, value):
       self._x = value

    x = property(x_getter, x_setter)   # wrap both getter and setter in one call

Here,

class Test:
    x = property(
        lambda self: getattr(self, "_x"),  # getter
        lambda self, value: setattr(
            self, "_x", f"{value} From Setter"
        ),  # setter
    )

    def __init__(self):
        print("init called")
        self.x = "Hi"


test = Test()
test.x = "Hello"
print(test.x)

Output:

init called
Hello From Setter

The property class has three methods within it getter, setter, and deleter. You can guess by the name what they do. You can either use a init method to initialize whatever method you need or you can explicitly call the methods later and set the functions.

May I know why you wanna do that?