Explain what the PermissionRequiredMixin mixin in Django does and how it is used for access control.

PermissionRequiredMixin in Django is a mixin used in class-based views to control access based on user permissions. It checks whether the user has certain permissions before allowing access to the view.

To use PermissionRequiredMixin, you need to specify the permission_required attribute — a string or list of strings with the names of the required permissions, for example 'app_label.permission_codename'.

If the user lacks the necessary permissions, they will be redirected to the login page or receive a 403 error by default.

Example of usage:

from django.contrib.auth.mixins import PermissionRequiredMixin
from django.views.generic import View

class MyView(PermissionRequiredMixin, View):
    permission_required = 'myapp.view_secret_data'

    def get(self, request, *args, **kwargs):
        # view logic
        pass

Important: PermissionRequiredMixin should be listed first in the inheritance list to ensure its methods work correctly.