Decorating a property: right order

Question

Is there a preferred way of decorating a class property? As I see it you could either decorate the property itself with another property or you could decorate the underlying method and then apply @property on that.
Are there any considerations to make for either approach?

def decorate_property(prop):
    @property
    def inner(instance):
        return prop.__get__(instance) + 1
    return inner

def decorate_func(func):
    def inner(instance):
        return func(instance) +1
    return inner

class A:

    x = 1

    @decorate_property
    @property
    def f(self):
        return self.x

    @property
    @decorate_func
    def g(self):
        return self.x

a = A()
print(a.f) # 2
print(a.g) # 2

Check this out - https://www.programiz.com/python-programming/property — Justin, Apr 25 '19 at 11:15
This too - https://www.codementor.io/sheena/advanced-use-python-decorators-class-function-du107nxsv — Justin, Apr 25 '19 at 11:16
Possible duplicate of - https://stackoverflow.com/questions/2406567/preferred-way-of-defining-properties-in-python-property-decorator-or-lambda — Justin, Apr 25 '19 at 11:17

MisterMiyagi · Answer 1 · 2019-04-25T14:16:54.307

Properties are objects (descriptors) that wrap functions. Which one to decorate depends on which one you intend to change.

If you want to change what the property does, decorate the initial function.

@property
@plus_one  # add one to the result of the function
def a(self):
    return self._a

This also applies to changing the setter and deleter.

@property
@plus_one  # add one to the result of the function
def a(self):
    return self._a

@a.setter
@minus_one  # remove one from the input of the function
def a(self, value):
    self._a = value

If you want to change what the property is, decorate the resulting descriptor.

@random_repr  # add a new repr to the descriptor
@property
def a(self):
    return 1

Note that defining setter and deleter creates a new descriptor. You need to decorate the last descriptor created.

@property
def a(self):
    return self._a

@random_repr  # add a new repr to the final descriptor
@a.setter
def a(self, value):
    self._a = value

For the most part, a property is used to get a certain behaviour expressed by a function (case 1). The data descriptor (case 2) is an implementation detail that most people are not familiar with. In case of doubt, decorate the function - this case is the more generic and easier to understand.

Decorating a property: right order

1 Answers1