Python property() — What You Always Wanted to Know But Never Dared to Ask

Object-orientation is great way to encapsulate data in your application. This minimizes complexity and adheres to good software engineering principles.

However, attributes in Python can be easily accessed from the outside—they’re not really encapsulated. That’s one of the reason the property() built-in function exists: it allows you to truly encapsulate data with the means of private attributes that you can access via getter and setter functions from the outside.

Python property()

Python’s built-in property() function creates and returns a new property attribute that should be private, i.e., only accessible via defined functions. As arguments, you pass three functions to get, set, and delete the attribute value—as well as the fourth docstring argument. All four arguments are None per default.

Here’s the formal syntax and argument list of the property() function:

property(fget=None, fset=None, fdel=None, doc=None)
ArgumentsfgetCallable function object for getting an attribute value. Returns the attribute value.
fsetCallable function object for setting an attribute value. No return value required.
fdelCallable function object for deleting an attribute value. No return value required.
docString describing the documentation of the attribute.
Return ValuepropertyReturns new object of class property that allows you to access the property from the outside via defined getter and setter methods.

Python property() — Usage Examples

Learn by example! In the following, we’re going to explore an example of why and how to use the property() built-in function.

Have a look at this simple Car class for which we create a “secret” _owner attribute—note the prefixed underscore to discourage external access:

class Car:

    def __init__(self):
        self._owner = None

    def get_owner(self):
        return self._owner

    def set_owner(self, name):
        self._owner = name

    def delete_owner(self):
        del self._owner


porsche = Car()
porsche.set_owner('Chris')
print(porsche.get_owner())
# Chris

The owner attribute has a getter function, a setter function, and a delete function that removes the attribute.

This follows the guidelines of object orientation to allow external access of object attributes only via functions rather than directly via porsche._owner. However, using complicated getter and setter function names can be a pain. That’s why you can also add an additional line using the property() function to make _owner a property rather than just an attribute:

class Car:

    def __init__(self):
        self._owner = None

    def get_owner(self):
        return self._owner

    def set_owner(self, name):
        self._owner = name

    def delete_owner(self):
        del self._owner

    # Creates a property: Car.owner
    owner = property(get_owner,
                     set_owner,
                     delete_owner,
                     'Owner of the car')
    

porsche = Car()
porsche.owner = 'Chris'
print(porsche.owner)
# Chris

This greatly facilitates access of the attribute. Instead of calling the clunky porsche._owner or even porsche.get_owner(), you can now call porsche.owner as if owner was an attribute. Python then internally calls the defined getter and setter functions to modify, access, or delete the attribute _owner.

Python property() — Video


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Python property() — Interactive Code Exercise

Let’s have a look at a practical exercise:

Exercise: Replace both occurrences of ??? with some code to make you the owner of the Porsche!


Summary

Object-orientation is great way to encapsulate data in your application. This minimizes complexity and adheres to good software engineering principles.

However, attributes in Python can be easily accessed from the outside—they’re not really encapsulated. That’s one of the reason the property() built-in function exists: it allows you to truly encapsulate data with the means of private attributes that you can access via getter and setter functions from the outside.

Python’s built-in property() function creates and returns a new property attribute that should be private, i.e., only accessible via defined functions. As arguments, you pass three functions to get, set, and delete the attribute value—as well as the fourth docstring argument. All four arguments are None per default.


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