What does Django's @property do?

Question

What is @property in Django?

Here is how I understand it: @property is a decorator for methods in a class that gets the value in the method.

But, as I understand it, I can just call the method like normal and it will get it. So I am not sure what exactly it does.

Example from the docs:

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    birth_date = models.DateField()

    def baby_boomer_status(self):
        "Returns the person's baby-boomer status."
        import datetime
        if self.birth_date < datetime.date(1945, 8, 1):
            return "Pre-boomer"
        elif self.birth_date < datetime.date(1965, 1, 1):
            return "Baby boomer"
        else:
            return "Post-boomer"

    @property
    def full_name(self):
        "Returns the person's full name."
        return '%s %s' % (self.first_name, self.last_name)

What is the difference of if it is there vs if it isn't?

Answer 1

As you see, the function full_name returns a string with the persons first and last name.

What the @property decorator does, is declare that it can be accessed like it's a regular property.

This means you can call full_name as if it were a member variable instead of a function, so like this:

name = person.full_name

instead of

name = person.full_name()

You could also define a setter method like this:

@full_name.setter
def full_name(self, value):
     names = value.split(' ')
     self.first_name = names[0]
     self.last_name = names[1]

Using this method, you can set a persons full name like this:

person.full_name = 'John Doe'

instead of

person.set_full_name('John Doe')

P.S. the setter above is just an example, as it only works for names that consist of two words separated by a whitespace. In real life, you'd use a more robust function.

Answer 2

In some languages users are encouraged to make attributes private and create public getter and setter methods, e.g. in some made up Python-like language with private and public:

class Foo:
    private bar

    public get_bar(bar):
        return self.bar  # or look it up in a database
                         # or compute it on the fly from other values
                         # or anything else

    public set_bar(new_bar):
        self.bar = new_bar

The argument is about providing a stable interface. If you want to change the inner workings of your class, e.g. to look it up from a database or compute it, users of the class won't have to change anything; they just keep calling the getter and setter.

In Python we don't really have private attributes, and we want simple syntax. So we flip it: programmers often directly access an object's attributes. But what if you want to change the internal behaviour? We don't want to change the class' interface.

@property lets you change how bar works internally without changing the external interface. Users of the class can still access foo.bar, but your internal logic can be completely different:

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

def main():
    f = Foo()
    print(f.bar)

# Later we can change to something like this without breaking other code
class Foo:
    def __init__(self, bar):
        self.save_bar_to_database(bar)  # Or anything else

    @property
    def bar(self):
        return self.load_bar_from_database()

Answer 3

It is a simple way, where you can get variables in the table and provide another variable which can be used directly as it was a variable.

@property
def total(self):
    total_price = self.products.price_each * self.quantity_prt
    return total_price 

such as abouve function you can get number of products and price and ue a property and make a variable of total price.