Django URL pattern using pk error

Question

I'm using the latest version of python3 and Django2, trying to do an URL pattern that is dynamic and changes for every different variable, Here are the codes:

urls.py

path('categories/<int:item_category>/', views.item_category, name="item_category"),

views.py

def item_category(request, pk):
    item_category = get_object_or_404(Categories, pk=pk)

    return render(request, 'items_modal.html', {'item_category': item_category})

models.py

class Categories(models.Model):
    category_name  = models.CharField(max_length=30)

    def __str__(self):
        return self.category_name

    def item_category(self):
        return reverse('item_category', args=[self.pk])

home.html

    <div class="table-responsive">
   <table class="table table-hover">
  <thead class="thead-dark">
    <tr>
      <th scope="col"><h2 align="center"> محتويات المخزن</h2></th>
    </tr>
  </thead>
  <tbody>
     {% for cat in all_cats %}
    <tr>
      <th scope="row"><a href="{% url 'item_category' item_category.pk %}"" data-toggle="modal" data-target="#exampleModal">{{ cat }}</a></th>

    </tr>
    {% endfor %}
  </tbody>
</table>
</div>

when I try to go to open the home page it gives me the error :

NoReverseMatch at /
Reverse for 'item_category' with arguments '('',)' not found. 1 pattern(s) tried: ['categories\\/(?P<item_category>[0-9]+)\\/$']
Request Method: GET
Request URL:    http://127.0.0.1:8000/
Django Version: 2.0.4
Exception Type: NoReverseMatch
Exception Value:    
Reverse for 'item_category' with arguments '('',)' not found. 1 pattern(s) tried: ['categories\\/(?P<item_category>[0-9]+)\\/$']
Exception Location: C:\Users\Dev3\AppData\Local\Programs\Python\Python36-32\lib\site-packages\django\urls\resolvers.py in _reverse_with_prefix, line 632
Python Executable:  C:\Users\Dev3\AppData\Local\Programs\Python\Python36-32\python.exe
Python Version: 3.6.5
Python Path:    
['C:\\python\\Django\\nothing',
 'C:\\Users\\Dev3\\AppData\\Local\\Programs\\Python\\Python36-32\\python36.zip',
 'C:\\Users\\Dev3\\AppData\\Local\\Programs\\Python\\Python36-32\\DLLs',
 'C:\\Users\\Dev3\\AppData\\Local\\Programs\\Python\\Python36-32\\lib',
 'C:\\Users\\Dev3\\AppData\\Local\\Programs\\Python\\Python36-32',
 'C:\\Users\\Dev3\\AppData\\Local\\Programs\\Python\\Python36-32\\lib\\site-packages']
Server time:    Tue, 24 Apr 2018 09:39:46 +0000

As an aside, it's recommended to use the singular for model names (e.g. Category instead of Categories. Also I would avoid using the same variable name for the view item_category and the category instance inside the view. — Alasdair
– Alasdair, Commented Apr 24, 2018 at 10:03

Marcos Aguayo · Accepted Answer · 2018-04-24 10:17:16Z

2

Reverse for 'item_category' with arguments '('',)' not found. 1 pattern(s) tried: ['categories\\/(?P<item_category>[0-9]+)\\/$']

In the error message, with arguments '('',)' is telling you that the argument in the url tag evaluated to the empty string ''.

Looking at your template, you loop through {% for cat in all_cats %} but then you use item_category.pk inside the loop. You probably want cat.pk:

{% for cat in all_cats %}
<tr>
  <th scope="row"><a href="{% url 'item_category' cat.pk %}" data-toggle="modal" data-target="#exampleModal">{{ cat }}</a></th>
</tr>
{% endfor %}

edited Apr 24, 2018 at 10:17

Marcos Aguayo

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answered Apr 24, 2018 at 9:52

Alasdair

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4 Comments

Ahmed Wagdi Over a year ago

and it worked like charm. Thank you @Alasdair Don't you know how to use that keyword in the output page as a variable, I want to use this cat.pk for the link the user clicked as a filter in the output page

Alasdair Over a year ago

I don't understand your question. You already use the pk in the item_category view: item_category = get_object_or_404(Categories, pk=pk).

Ahmed Wagdi Over a year ago

When the user clicks on a text(the {{ cat }} variable i used in HTML ) it take him to the URL /categories/(pk)/ , i want to use the clicked {{ cat }} as <h1> title in the new opened page

Alasdair Over a year ago

In that view, you are using the context {'item_category': item_category}. Therefore you can use {{ item_category }} in the template for that view. If you're still stuck, please ask a new question because this is a separate issue.

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