Trouble to display DELETE and PUT action in Django Rest

I am sure that I follow instructions step by step in tutorial 4. However, I am not able to display the DELETE and PUT in the page and in Allows attribute. It seems that Django rest ignore the IsOwnerOrReadOnly permission. Can I anyone help me to debug it?

In models.py

from django.db import models
from pygments.lexers import get_all_lexers
from pygments.styles import get_all_styles
from pygments.lexers import get_lexer_by_name
from pygments.formatters.html import HtmlFormatter
from pygments import highlight


LEXERS = [item for item in get_all_lexers() if item[1]]
LANGUAGE_CHOICES = sorted([(item[1][0], item[0]) for item in LEXERS])
STYLE_CHOICES = sorted((item, item) for item in get_all_styles())


class Snippet(models.Model):
  created = models.DateTimeField(auto_now_add=True)
  title = models.CharField(max_length=100, blank=True, default='')
  code = models.TextField()
  linenos = models.BooleanField(default=False)
  language = models.CharField(choices=LANGUAGE_CHOICES, default='python', max_length=100)
  style = models.CharField(choices=STYLE_CHOICES, default='friendly', max_length=100)
  owner = models.ForeignKey('auth.User', related_name='snippets')
  highlighted = models.TextField()

  def save(self, *args, **kwargs):
    """
    Use the `pygments` library to create a highlighted HTML
    representation of the code snippet.
    """
    lexer = get_lexer_by_name(self.language)
    linenos = self.linenos and 'table' or False
    options = self.title and {'title': self.title} or {}
    formatter = HtmlFormatter(style=self.style, linenos=linenos,
                              full=True, **options)
    self.highlighted = highlight(self.code, lexer, formatter)
    super(Snippet, self).save(*args, **kwargs)

  class Meta:
    ordering = ('created',) 

In serializers.py

from rest_framework import serializers
from snippets.models import Snippet, LANGUAGE_CHOICES, STYLE_CHOICES
from django.contrib.auth.models import User

class SnippetSerializer(serializers.ModelSerializer):

    owner = serializers.ReadOnlyField(source='owner.useame')
    class Meta:
        model = Snippet
        fields = ('id', 'title', 'code', 'linenos', 'language', 'style','owner')


class UserSerializer(serializers.ModelSerializer):
    snippets = serializers.PrimaryKeyRelatedField(many=True, queryset=Snippet.objects.all())

    class Meta:
        model = User
        fields = ('id', 'useame', 'snippets')

In views.py

from django.http import HttpResponse
from django.views.decorators.csrf import csrf_exempt
from rest_framework.renderers import JSONRenderer
from rest_framework.parsers import JSONParser
from snippets.models import Snippet
from snippets.serializers import SnippetSerializer
from rest_framework.response import Response
from rest_framework.decorators import api_view
from django.http import Http404
from rest_framework.views import APIView
from rest_framework import status
from rest_framework import mixins
from rest_framework import generics
from django.contrib.auth.models import User
from snippets.serializers import UserSerializer
from rest_framework import permissions
from snippets.permissions import IsOwnerOrReadOnly



class UserList(generics.ListAPIView):
    queryset = User.objects.all()
    serializer_class = UserSerializer

    def perform_create(self, serializer):
        serializer.save(owner=self.request.user)

    permission_classes = (permissions.IsAuthenticatedOrReadOnly,)


class UserDetail(generics.RetrieveAPIView):
    queryset = User.objects.all()
    serializer_class = UserSerializer

permission_classes = (permissions.IsAuthenticatedOrReadOnly,
                      IsOwnerOrReadOnly,)

In permissions.py

from rest_framework import permissions


class IsOwnerOrReadOnly(permissions.BasePermission):
    """
    Custom permission to only allow owners of an object to edit it.
    """

    def has_object_permission(self, request, view, obj):
        # Read permissions are allowed to any request,
        # so we'll always allow GET, HEAD or OPTIONS requests.
        if request.method in permissions.SAFE_METHODS:
            retu True

        # Write permissions are only allowed to the owner of the snippet.
        retu obj.owner == request.user