Python Object Property Control

Accessors and Mutators, and how @property is your friend

Basic Concepts

properties of an object (basically internal variables to the object) should usually not be directly accessible
Properties should also be able to internally consistent (if you change the length of a box, it's volume should change etc)
Many languages have ways to control how you set/modify properties
- To retrieve/access object properties you use an Accessor, often called "getters" for getting the value
- To change the value on a property, you use a Mutator, or function that is designed to set the value (a setter)
Python has several ways of doing so
- when ACCESSING a property, (aka get the value) the @property decorator internally calls a function that MUST return a value. From the user perspective, it just seems like a normal property ie box1.width, there is no hint that it is actually a function
- when MUTATING a property (aka set, or change the value) the @property decorator calls another function, and you decide how/if to change the value of the real property box1._width, but the user just does the normal box1.width = 23.5
- There is nothing preventing the user from directly manipulating the real property box1._width other than convention (and that when folks use your module, @property protected properties do not show up in the IDE)

python object limitations

There are no true "private" properties or methods, nor are there any real "constants"
There is a CONVENTION that says "don't mess with properties that start with a" _ ie self._weight
But you can still do objectname._weight = "bob"

Options for Controlling access to Object Properties

Note, ALL of these still use the self._propertyname convention, which is basically a polite agreement among programmers

Creating specific set and get functions

Considered bad practice, yet not unusual to see them now and then
for example, see this class box object example that has getHeight() and setHeight() methods
a bit clunky

`property()` function

property() is one of the powerful Python built-in functions
Used inside the class definition where you have a self._height would be height = property(getHeight,setHeight,delHeight,documentstring)

All of these are optional functions/string at end so, to alter the class box example (see link above)

class MakeBox:
    def __init__(self, length, width, height, density):
        self._length = length
        self._width = width
        self._height = height
        self._density = density
        self.reCalc()

    def getVolume(self):
        self._volume = self._width * self._length * self._height
        return self._volume

    volume = property(getVolume)

    def getSurface(self):
        step1 = self._width * self._height * 2
        step2 = self._width * self._length * 2
        step3 = self._height * self._length * 2
        self._surface = step1 + step2 + step3
        return self._surface

    surface = property(getSurface)

    def getWeight(self):
        self._weight = self._volume * self._density
        return self._weight

    weight = property(getWeight)

    def reCalc(self):
        self.getVolume()
        self.getSurface()
        self.getWeight()

    # Should have a set height,width,length, density (but not for volume,surface,weight as they are derived from the first four)
    # should have a get height, width, length, density, volume, surface, weight
    # you may also want to rename the properties with an _ to let folks know NOT to access them directly

    def setHeight(self,newHeight):
        self._height = newHeight
        self.reCalc()

    def getHeight(self):
        return self._height

    def printstats(self):
        # should not be done this way in class
        print(f"""
        Box Height: {self._height}
        Box Width: {self._width}
        Box Length: {self._length}
        Box Density: {self._density}
        Box Volume: {self._volume}
        Box Weight: {self._weight}
        Box Surface Area: {self._surface}
        """)

    height = property(getHeight,setHeight)

if __name__ == "__main__":
    box1 = MakeBox(12,34,23,2.3)

    box1.printstats()
    print(box1.height)
    box1.height = 32
    box1.printstats()
    print(box1.height)

    print(box1.volume)    
    print(box1.surface)
    print(box1.weight)

    # this errors out as no "setter" function has been specified
    # box1.weight = 350

`@property` decorator

wraps a version of the property() function around the function that follows right afterwards (this will be your "getter")
so if you write @property followed by def timeleft(): your actual property will be self._timeleft
For setters, you use the property name (minus the _) and then .setter so @timeleft.setter followed by the function def timeleft():
This seems like duplicate names, but each is "wrapped" inside the decorator

Once again re writing the above code:

class MakeBox:

    """
    If I were to add @property/.setter for length, width, density, I could remove the "_" from those properties inside the
    calculations for Volume, Weight, Surface area.
    """

    def __init__(self, length, width, height, density):
        self._length = length
        self._width = width
        self._height = height
        self._density = density

    @property
    def volume(self):
        self._volume = self._width * self._length * self.height
        return self._volume

    @property
    def surface(self):
        step1 = self._width * self.height * 2
        step2 = self._width * self._length * 2
        step3 = self.height * self._length * 2
        self._surface = step1 + step2 + step3
        return self._surface

    @property
    def weight(self):
        self._weight = self.volume * self._density
        return self._weight

    # Should have a set height,width,length, density (but not for volume,surface,weight as they are derived from the first four)
    # should have a get height, width, length, density, volume, surface, weight
    # you may also want to rename the properties with an _ to let folks know NOT to access them directly

    @property
    def height(self):
        return self._height

    @height.setter
    def height(self,newHeight):
        self._height = newHeight

    def printstats(self):
        # should not be done this way in class
        print(f"""
        Box Height: {self.height}
        NOTE: Following do not have @property set yet, so still using the ._name
        Box Width: {self._width}
        Box Length: {self._length}
        Box Density: {self._density}
        Note: These DO have @property, but no .setter, so they can be used as properties
        Box Volume: {self.volume}
        Box Weight: {self.weight}
        Box Surface Area: {self.surface}
        """)

if __name__ == "__main__":
    box1 = MakeBox(12,34,23,2.3)

    box1.printstats()
    print(box1.height)
    box1.height = 32
    box1.printstats()
    print(box1.height)

    print(box1.volume)
    print(box1.surface)
    print(box1.weight)

Basic Concepts

python object limitations

Options for Controlling access to Object Properties

Creating specific set and get functions

property() function

@property decorator

Resources

`property()` function

`@property` decorator