In this tutorial, we are going to discuss the for built-in tag in Django Template System.
The for tag corresponds to the for looping construct statement, which is to execute statements in a loop(again and again) while iterating over a sequence of values.
Working of for tag
The for tag allows us to loop over a collection of items in a sequence, starting from the first item in a sequence to
the last item in a sequence.
The for tag starts with the {% for %} tag and ends with a {% endfor %} tag. All the texts between the start and end tag is known as a
block. Let us see a sample code with for tag to display a list of fruits provided in fruits:
{% for x in fruits %}
<p>{{ x }}</p>
{% endfor % }
As you can see in the code above, the {% for %} tag iterates over a sequence of values referenced by a variable - fruits i.e. iteration starts from the first fruit item in a sequence and ends at the last fruit item in a sequence.
For
each iteration, the variable x will be assigned the next fruit item in the sequence and the Django template system will display everything between {% for %} and {% endfor %} block, where the {% endfor %} block marks the end of an for template tag.
Note: In the sample code above, <p> correspond to the paragraph HTML tag.
Variables of for tag
The for tag gives some variables, each for a different purpose. Let us discuss these variables.
Variables
Description
forloop.counter
The forloop.counter variable gives us the total number of items we have already iterated over using the for tag.
It is one-indexed.
forloop.counter0
The forloop.counter0 variable gives us the total number of items we have already iterated over using the for tag.
It is zero-indexed.
forloop.first
The forloop.first variable gives us a boolean value True, if we are iterating over a collection
of items for the first time using the for tag.
forloop.last
The forloop.last variable gives us a boolean value True, if we are iterating over a collection of
items for the last time using the for tag.
forloop.revcounter
The forloop.revcounter variable gives a total number of leftover items to be iterated over using
the for tag.
It is one-indexed.
forloop.revcounter0
The forloop.revcounter variable gives a total number of leftover items to be iterated over using
the for tag.
It is zero-indexed.
Add reversed to for tag
We could add reversed keyword with the for tag to reverse the iteration over a sequence of items.
Django project with for tag
Up next, we are going to explain a situation is which the Django Template System encounters the for tag in an executing HTML template file.
For those who don't know how to create a Django Project from a scratch and have not read our previous tutorials, here is a recap.
Creating a Django project
To create a directory for our project, we need to open the Command Prompt and type in the command -
django-admin startproject django_proj1
dhago-admin is a management utility for Django Framework, using which we could create a new project and much more.
startproject is a command to start a new Django project.
django_proj1 is the name of our Django project and also the name of our project directory created in the current directory.
The startproject command gives us a sub-directory and files
Executing the startproject command has not only created a project directory named django_proj1, but has also created a file manage.py and another sub-directory
with the same name as the project directory, which also contains some Python files -
django_proj1 is the root directory of our Django project, also known as the container of our project.
manage.py is a command-line utility which allows us to interact with Django project in various ways.
django_proj1 sub-directory is actually a Python package for our project. We can import this package(a group of modules) and its contents just like we import any general Python package/module and its contents.
__init__.py is a file which allows our sub-directory django_proj1 to be treated as a Python package.
settings.py is a file used for setting the configurations for our Django project.
urls.py is a file used to specify the URL declarations for our Django project, also known as a a table of contents.
wsgi.py is a file used as an entry-point for WSGI web-servers to serve your project.
Starting the development server
After creating the project directory and the files required to run our Django project, it is time to execute the Django Development Server. To do this,
we will have to open the command prompt, change the current directory to the root directory of our Django project and execute the following command.
python manage.py runserver
Executing the above mentioned command will trigger the Django Development Server in action, as you can see in the picture below.
Note: Please ignore the warning of unapplied database migrations for now, we will talk more about it later in the section - Database with Django.
Now that you have successfully started the Django development server, it is time to visit the http://127.0.0.1:8000/ link and you will see a webpage displaying a congratulatory message, as shown below.
Note: The runserver command runs the Django development server at the port 8000.
Creating an app in our project
A project may contain one or multiple applications. As we have already created a project, now let us create an application which compares the values of two variables using the for tag of Django Template System. This application will be created right under our root - project directory django_proj1.
To create an application, we have to open the command prompt, change the current directory to the root directory of our Django project and execute the following command -
python manage.py startapp compare
The command startapp in combination with manage.py utility is used to create an app of a project, using which we have created an app named - compare.
Executing the startapp command has not only created an application directory named compare in the root - project directory, but has also created the following set of Python files -
Next, we are going to create an HTML template file for this application.
Creating an HTML template file for our app.
Next, we are going to create an HTML template file within our application folder compare, we will name this file template.htm.
template.htm
<html>
<head><title>Django Template System - Decodejava.com</title></head>
<body>
Looping over a sequence of items using <b>forloop.counter</b> variable:
<br/>
{% for x in list2 %}
<i>{{ forloop.counter }} </i> : {{ x }}<br>
{% endfor %}
<br/>
Looping over a sequence of items using <b>forloop.counter0</b> variable:
<br/>
{% for x in list2 %}
<i>{{ forloop.counter0 }}</i> : {{ x }}<br/>
{% endfor %}
<br/>
Printing the first element of sequence using <b>forloop.first</b> variable:
<br/>
{% for x in list2 %}
{% if forloop.first %}
<i>The first item</i> - {{ x }}
{% endif%}
{% endfor %}
<br/>
<br/>
Printing the last element of sequence using <b>forloop.last</b> variable:
<br/>
{% for x in list2 %}
{% if forloop.last %}
<i>The last item</i> - {{ x }}
{% endif%}
{% endfor %}
<br/>
<br/>
Printing the left over items in the sequence using <b>forloop.revcounter</b> variable:
<br/>
{% for x in list2 %}
<i>Total number of leftover item to be iterated</i> - {{ forloop.revcounter }}<br/>
<i>Item</i> - {{ x }}<br/>
{% endfor %}
<br/>
Printing the left over items in the sequence using <b>forloop.revcounter0</b> variable:
<br/>
{% for x in list2 %}
<i>Total number of leftover item to be iterated</i> - {{ forloop.revcounter0 }}<br/>
<i>Item</i> - {{ x }}<br/>
{% endfor %}
<br/>
Printing the reversed sequence of items using <b>reversed</b> attribute of <b>for</b> tag: <br/>
{% for x in list2 reversed %}
<i>Item</i> - {{ x }}<br/>
{% endfor %}
</body>
</html>
In this HTML template file, we are using for template tag to iterate over a list, which is defined in the view function(we are going to create it next). In this HTML template file, besides using for tag, we have also used its variables - forloop.counter, forloop.counter0, forloop.first, forloop.last. forloop.revcounter and forloop.revcounter0.
Creating a view function for the app
Once we have created the HTML template file in the application, we will also need to create a view function associated with it. A view function is simply a Python function which takes an HttpRequest from the HTML template file and returns an HttpResponse with the values of the variables specified and requested by the HTML template file.
Note: This view function is defined in the file view.py, which is already created and stored in our application directory, compare. Let us just copy and paste the following code in view.py.
We have named this view function - tag_lookup().
view.py
from django.shortcuts import render
import datetime
def tag_lookup(request):
#Creating a dictionary of key-value pairs
dic = { 'list2': [1, datetime.date(2019,7,16), 'Do not give up!'] }
#Calling the render() method to render the request from template_ex.htm page by using the dictionary, dic
return render(request, "template_ex.htm", dic)
Understanding the code in view.py :
First, we have imported the render function from the django.shortcuts module and next we have imported the datetime module.
Next, we have defined a view function named - templ, which takes an HttpRequest object in its parameter and we have named this parameter - request.
Note:Each view function takes an HttpRequest object in its first parameter and it could named anything but its typically named - request.
In the view function, we have simply created a dictionary(dic) of items of a key:value pair, where the key refers to a list collection of items.
Note:In the HTML template file, each key is used as a variable name to access the its value specified in the dictionary(dic).
To initiate the process of rendering, we call the render() method, which replaces variables in HTML template file with their values, which are looked up in the Context(a parameter to the render() method).
The render() method requires three parameters passed to it:
Request - An incoming request.
Path to the HTML template - This is a path relative to the path specified in settings.py file.
Context - Since, Django 1.8+ version, the render() method takes a Context, which should be represented by a dictionary of items needed in the template.
On call, the render() method renders the request from template_ex.htm page by using the items of dictionary(dic), and it has returned an HttpResponse object with the rendered text, which is then eventually returned back by the view function - tag_lookup().
Mapping URL to view
After creating the view function of an application, we need to specify the mapping between a URL and this view function in a file named - urls.py. The file urls.py is simply coded in Python and we will have to create the urls.py file in the folder of our application, compare.
The use of mapping a URL to the view function is, when this URL is requested by the user, its mapped view function is executed and a response is returned.
urls.py
from django.urls import path
from . import views
urlpatterns = [
path('', views.tag_lookup),
]
As you can see in the file urls.py, we have imported the path() function from the module django.urls.
The path() function has been passed the two must arguments -
An empty string to specify the URL pattern, specified by ' '.
This URL pattern specified in the application's urls.py file is the relative path to the template. This path gets added to another URL pattern, specified in the project's urls.py file(we are going to create it next), and the combined path will be matched to the view function - tag_lookup defined in the application's views.py.
Pointing the project url to application url
Next, we are going to point the project url(urls.py) to the application url(urls.py) by using the include() function. For this, we will open the file urls.py in the root - project directory, django_proj1, and copy-paste the following code.
urls.py
from django.contrib import admin
from django.urls import include, path
urlpatterns = [
path('tags', include('compare.urls')),
]
As you can see in the file urls.py, we have imported the path() and include() function from the module django.urls.
The path() function has been passed the two must arguments -
A string to specify the URL pattern - 'tags/'. This is the relative path to the template.
This URL pattern, combined with the URL pattern defined in the application's urls.py will be matched to the view function - tag_lookup, which is specified in the application urls.py file( stored in the application directory compare). Therefore we have used the include() function to include and refer to the application's urls.py file.
How to load the HTML template file.
Before we finally execute the project, we must load the above created HTML template template.htm, which is stored within our application folder compare.
To load the HTML template file, we need to specify its full path in the configuration file - settings.py, which is stored in the project folder. For our example, you can find this file in the sub-directory django_proj1 within our project django_proj1 i.e. - django_proj1/django_proj1/settings.py.
Within the settings.py file, you need to locate the property DIRS and copy-paste the full-path to the HTML template file against it, as shown below and save the file.
'DIRS': ['E:/django_proj1/compare/'],
Executing the application of our project
To execute the compare application of our project django_proj1, we just have to enter the address - http://127.0.0.1:8000/tags/.
As you can see in the picture, the string tags/ in the URL is the URL-pattern which gets matched to the URL pattern specified in the project urls.py file, which calls the application's urls.py and its associated view function - tag_lookup(in the application) is executed and its response is displayed.
The response displays the result of using the for template tag in HTML Template file to iterate over a list defined in the view function.
In this HTML template file, besides using for tag, we have also used its variables - forloop.counter, forloop.counter0, forloop.first, forloop.last. forloop.revcounter and forloop.revcounter0.