Usefulness of property decorator in python for mutable attributes

Question

Trying to understand the benefit of properties in Python when we need to be able to set them. Explicitly looking to see the pros/cons of the two techniques.

What are the benefits of using the @property with @prop.setter in Python:

class Foo2:

    def __init__(self, prop):
        self.prop = prop

    @property
    def prop(self):
        return self._prop

    @prop.setter
    def prop(self, value):
        self._prop = value

Compared to simply setting the attribute without the property decorator?

class Foo:
    def __init__(self, prop):
        self.prop = prop

Possible duplicate of [Real world example about how to use property feature in python?](https://stackoverflow.com/questions/6304040/real-world-example-about-how-to-use-property-feature-in-python) — Josh Karpel, Jun 15 '19 at 18:01

score 2 · Accepted Answer · answered Jun 15 '19 at 18:04

You created an attribute that can't be deleted:

>>> f = Foo2('bar')
>>> f.prop
'bar'
>>> f.prop = 'spam'
>>> f.prop
'spam'
>>> del f.prop
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: can't delete attribute

Compare this to the Foo().prop attribute, which can be deleted:

>>> f = Foo('bar')
>>> f.prop
'bar'
>>> del f.prop
>>> f.prop
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Foo' object has no attribute 'prop'

Apart from that, or if you added a @prop.deleter handler, there is not really any advantage to creating such a property. Delegating to a function what normal attribute access can do is not that useful.

Property setters or getters are much more useful when they do something in addition to just setting the attribute, like validation or transformation.

For example, the setter could enforce that values are always integers, including setting a default value if conversion to an integer fails:

@prop.setter
def prop(self, value):
    try:
        self._prop = int(prop)
    except ValueError:
        self._prop = 0

In other programming languages such as Java, you can't easily convert attributes to getters and setters (there is no equivalent concept to Python properties in Java) without having to re-write all access to those attributes everywhere in your code-base, so in those languages you often see advice to start out with getters and setters. This doesn't apply to Python, you can trivially turn existing attributes into properties without having to change any code using those attributes.

Usefulness of property decorator in python for mutable attributes

1 Answers1