Restrict django choice field according to user

i have two users with two different permissions user a can create and close where usertype b can accept and complete i have to restrict the choice fields according to the user i am getting the error 

models.py

class Ticket(models.Model):

ticket_title = models.CharField(max_length=200)

ticket_description = models.TextField()

created_by = models.ForeignKey(User,related_name = 'created_by',blank=True,null=True,on_delete=models.CASCADE)

STATUS_CHOICES = (

('Opened','Opened'),

('Accepted','Accepted'),

('Completed','Completed'),

('Closed','Closed')

)

status = models.CharField('Status',choices=STATUS_CHOICES,max_length = 100,default = 'Opened')

accepted_by = models.ForeignKey(User,related_name='assigned_to',on_delete=models.CASCADE,blank=True,null=True)

def __str__(self):

return self.ticket_title

forms.py

class TicketEditForm(forms.ModelForm):

class Meta:

model = Ticket

fields = ('ticket_title', 'ticket_description','status')

def get_form_kwargs(self):

kwargs = super().get_form_kwargs()

kwargs['request'] = self.request.user

return kwargs

def __init__(self, *args, **kwargs):

usertype = kwargs.pop('request')

super().__init__(args, **kwargs)

if usertype.is_manager == True:

self.fields['status'].choices = (

('Closed','Closed'),

)

else:

self.fields['status'].choices = (

('Accepted','Accepted'),

('Completed','Completed'),

)

views.py

class DeveloperEditTicketView(LoginRequiredMixin,UpdateView):

template_name = 'app/edit_ticket.html'

model = Ticket

form_class = TicketEditForm

context_object_name = 'tickets'

success_url = reverse_lazy('dev_dashboard')

def form_valid(self,form):

ticket = form.instance

ticket.accepted_by = self.request.user

ticket.save()

return super().form_valid(form)

I am getting error as below

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