How can I write my own decorator in Django?

My models.py file is as follow:

from django.contrib.auth.models import User

class Shopkeeper(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE, primary_key=True)
    # ...


class Customer(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE, primary_key=True)
# ...

And I have some views which only Customers can access after login, but Shopkeepers cannot. And vice versa. How can I write decorator for such task?

There is nothing magical about a decorator, it is a function that takes as input the function (or class) to decorate, and makes some changes to it. If we look at the login_required decorator [GitHub], we see:

def login_required(function=None, redirect_field_name=REDIRECT_FIELD_NAME, login_url=None):
    """
    Decorator for views that checks that the user is logged in, redirecting
    to the log-in page if necessary.
    """
    actual_decorator = user_passes_test(
        lambda u: u.is_authenticated,
        login_url=login_url,
        redirect_field_name=redirect_field_name
    )
    if function:
        return actual_decorator(function)
    return actual_decorator

We can thus actually simply make a special case of the user_passes_test decorator:

from django.contrib.auth.decorators import user_passes_test

def shopkeeper_required(function=None):
    def is_shopkeeper(u):
        return Shopkeeper.objects.filter(user=u).exists()
    actual_decorator = user_passes_test(is_shopkeeper)
    if function:
        return actual_decorator(function)
    else:
        return actual_decorator

def customer_required(function=None):
    def is_customer(u):
        return Customer.objects.filter(user=u).exists()
    actual_decorator = user_passes_test(is_customer)
    if function:
        return actual_decorator(function)
    else:
        return actual_decorator

You can then for example implement it as:

@login_required
@shopkeeper_required
def some_shopkeeper_view(request):
    # ...
    pass

@login_required
@customer_required
def some_customer_view(request):
    # ...
    pass

Note that this @shopkeeper_required does not really enforces that the user is logged in, although in many cases that will be the case.

EDIT:

We can merge this with a @login_required (by adding a parameter that acts as a switch to turn this behavior on or off, by default on), like:

from django.contrib.auth.decorators import user_passes_test

def shopkeeper_required(function=None, login_required=True, redirect_field_name=REDIRECT_FIELD_NAME, login_url=None):
    def is_shopkeeper(u):
        if login_required and not u.is_authenticated:
            return False
        return Shopkeeper.objects.filter(user=u).exists()
    actual_decorator = user_passes_test(
        is_shopkeeper,
        login_url=login_url,
        redirect_field_name=redirect_field_name
    )
    if function:
        return actual_decorator(function)
    else:
        return actual_decorator

def customer_required(function=None, login_required=True, redirect_field_name=REDIRECT_FIELD_NAME, login_url=None):
    def is_customer(u):
        if login_required and not u.is_authenticated:
            return False
        return Customer.objects.filter(user=u).exists()
    actual_decorator = user_passes_test(
        is_customer,
        login_url=login_url,
        redirect_field_name=redirect_field_name
    )
    if function:
        return actual_decorator(function)
    else:
        return actual_decorator