Harmony/main/urls.py

from django.urls import path
from . import views

app_name = 'main'

urlpatterns = [
    path('callback/', views.callback, name='callback'),

    path('', views.product_list, name='product_list'),
    path('about/', views.about, name='about'),
    path('products/', views.product_list, name='product_list'),
    path('<slug:category_slug>/', views.product_list,
     name='product_list_by_category'),
     path('<int:id>/<slug:slug>/', views.product_detail,
     name='product_detail'),
    path('about/', views.about, name='about'),

]