Why would someone use @property if no setter or deleter are defined?

Question

In python code I often see the use of @property.

If I understand correctly, with the property function a getter setter and deleter can be defined.

Why would one use @property if the setter and deleter are not defined (@x.setter, @x.deleter)? Isn't this the same as not using @property at all?

Answer 1

It creates an API that does not allow a value to be set. This is similar in other languages to a constant.

Answer 2

Defining a property with a getter function but without a setter can be very useful in certain scenarios. Lets say you have a model as below in django; a model is essentially a database table with entries called fields. The property hostname is computed from one or more fields in the model from the database. This circumvents needing another entry in the database table that has to be changed everytime the relevant fields are changed.

The true benefit of using a property is calling object.hostname() vs. object.hostname. The latter is passed along with the object automatically so when we go to a place like a jinja template we can call object.hostname but calling object.hostname() will raise an error.

The example below is a virtualmachine model with a name field and an example of the jinja code where we passed a virtualmachine object.

# PYTHON CODE
class VirtualMachine(models.Model):
    name = models.CharField(max_length=128, unique=True)

    @property
    def hostname(self):
        return "{}-{}.{}".format(
            gethostname().split('.')[0],
            self.name,
            settings.EFFICIENT_DOMAIN
        )

# JINJA CODE
...start HTML...
Name: {{ object.name }}

# fails
Hostname: {{ object.hostname() }}

# passes
Hostname: {{ object.hostname }}
...end HTML...