oop - Real world example about how to use property feature in python?

Question

I am interested in how to use @property in Python. I've read the python docs and the example there, in my opinion, is just a toy code:

class C(object):
    def __init__(self):
        self._x = None

    @property
    def x(self):
        """I'm the 'x' property."""
        return self._x

    @x.setter
    def x(self, value):
        self._x = value

    @x.deleter
    def x(self):
        del self._x

I do not know what benefit(s) I can get from wrapping the _x filled with the property decorator. Why not just implement as:

class C(object):
    def __init__(self):
        self.x = None

I think, the property feature might be useful in some situations. But when? Could someone please give me some real-world examples?

Thanks.

Answer 1

Other examples would be validation/filtering of the set attributes (forcing them to be in bounds or acceptable) and lazy evaluation of complex or rapidly changing terms.

Complex calculation hidden behind an attribute:

class PDB_Calculator(object):
    ...
    @property
    def protein_folding_angle(self):
        # number crunching, remote server calls, etc
        # all results in an angle set in 'some_angle'
        # It could also reference a cache, remote or otherwise,
        # that holds the latest value for this angle
        return some_angle

>>> f = PDB_Calculator()
>>> angle = f.protein_folding_angle
>>> angle
44.33276

Validation:

class Pedometer(object)
    ...
    @property
    def stride_length(self):
        return self._stride_length

    @stride_length.setter
    def stride_length(self, value):
        if value > 10:
            raise ValueError("This pedometer is based on the human stride - a stride length above 10m is not supported")
        else:
            self._stride_length = value