How does the @property decorator work in Python?

Question

I would like to understand how the built-in function property works. What confuses me is that property can also be used as a decorator, but it only

Answer 1

A property can be declared in two ways.

• Creating the getter, setter methods for an attribute and then passing these as argument to property function
• Using the @property decorator.

You can have a look at few examples I have written about properties in python.

Answer 2

I read all the posts here and realized that we may need a real life example. Why, actually, we have @property? So, consider a Flask app where you use authentication system. You declare a model User in models.py:

class User(UserMixin, db.Model):
    __tablename__ = 'users'
    id = db.Column(db.Integer, primary_key=True)
    email = db.Column(db.String(64), unique=True, index=True)
    username = db.Column(db.String(64), unique=True, index=True)
    password_hash = db.Column(db.String(128))

    ...

    @property
    def password(self):
        raise AttributeError('password is not a readable attribute')

    @password.setter
    def password(self, password):
        self.password_hash = generate_password_hash(password)

    def verify_password(self, password):
        return check_password_hash(self.password_hash, password)

In this code we've "hidden" attribute password by using @property which triggers AttributeError assertion when you try to access it directly, while we used @property.setter to set the actual instance variable password_hash.

Now in auth/views.py we can instantiate a User with:

...
@auth.route('/register', methods=['GET', 'POST'])
def register():
    form = RegisterForm()
    if form.validate_on_submit():
        user = User(email=form.email.data,
                    username=form.username.data,
                    password=form.password.data)
        db.session.add(user)
        db.session.commit()
...

Notice attribute password that comes from a registration form when a user fills the form. Password confirmation happens on the front end with EqualTo('password', message='Passwords must match') (in case if you are wondering, but it's a different topic related Flask forms).

I hope this example will be useful

Answer 3

The property() function returns a special descriptor object:

>>> property()
<property object at 0x10ff07940>

It is this object that has extra methods:

>>> property().getter
<built-in method getter of property object at 0x10ff07998>
>>> property().setter
<built-in method setter of property object at 0x10ff07940>
>>> property().deleter
<built-in method deleter of property object at 0x10ff07998>

These act as decorators too. They return a new property object:

>>> property().getter(None)
<property object at 0x10ff079f0>

that is a copy of the old object, but with one of the functions replaced.

Remember, that the @decorator syntax is just syntactic sugar; the syntax:

@property
def foo(self): return self._foo

really means the same thing as

def foo(self): return self._foo
foo = property(foo)

so foo the function is replaced by property(foo), which we saw above is a special object. Then when you use @foo.setter(), what you are doing is call that property().setter method I showed you above, which returns a new copy of the property, but this time with the setter function replaced with the decorated method.

The following sequence also creates a full-on property, by using those decorator methods.

First we create some functions and a property object with just a getter:

>>> def getter(self): print('Get!')
... 
>>> def setter(self, value): print('Set to {!r}!'.format(value))
... 
>>> def deleter(self): print('Delete!')
... 
>>> prop = property(getter)
>>> prop.fget is getter
True
>>> prop.fset is None
True
>>> prop.fdel is None
True

Next we use the .setter() method to add a setter:

>>> prop = prop.setter(setter)
>>> prop.fget is getter
True
>>> prop.fset is setter
True
>>> prop.fdel is None
True

Last we add a deleter with the .deleter() method:

>>> prop = prop.deleter(deleter)
>>> prop.fget is getter
True
>>> prop.fset is setter
True
>>> prop.fdel is deleter
True

Last but not least, the property object acts as a descriptor object, so it has .__get__(), .__set__() and .__delete__() methods to hook into instance attribute getting, setting and deleting:

>>> class Foo: pass
... 
>>> prop.__get__(Foo(), Foo)
Get!
>>> prop.__set__(Foo(), 'bar')
Set to 'bar'!
>>> prop.__delete__(Foo())
Delete!

The Descriptor Howto includes a pure Python sample implementation of the property() type:

class Property:
    "Emulate PyProperty_Type() in Objects/descrobject.c"

    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        self.fget = fget
        self.fset = fset
        self.fdel = fdel
        if doc is None and fget is not None:
            doc = fget.__doc__
        self.__doc__ = doc

    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        if self.fget is None:
            raise AttributeError("unreadable attribute")
        return self.fget(obj)

    def __set__(self, obj, value):
        if self.fset is None:
            raise AttributeError("can't set attribute")
        self.fset(obj, value)

    def __delete__(self, obj):
        if self.fdel is None:
            raise AttributeError("can't delete attribute")
        self.fdel(obj)

    def getter(self, fget):
        return type(self)(fget, self.fset, self.fdel, self.__doc__)

    def setter(self, fset):
        return type(self)(self.fget, fset, self.fdel, self.__doc__)

    def deleter(self, fdel):
        return type(self)(self.fget, self.fset, fdel, self.__doc__)

Answer 4

The best explanation can be found here: Python @Property Explained – How to Use and When? (Full Examples) by Selva Prabhakaran | Posted on November 5, 2018

It helped me understand WHY not only HOW.

https://www.machinelearningplus.com/python/python-property/

Answer 5

Documentation says it's just a shortcut for creating readonly properties. So

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

is equivalent to

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

Answer 6

The first part is simple:

@property
def x(self): ...

is the same as

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

• which, in turn, is the simplified syntax for creating a property with just a getter.

The next step would be to extend this property with a setter and a deleter. And this happens with the appropriate methods:

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

returns a new property which inherits everything from the old x plus the given setter.

x.deleter works the same way.