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Django Template - if tag



In this tutorial, we are going to discuss the if built-in tag in Django Template System. The if tag corresponds to the if conditional statement which is used when we want to test a condition for a boolean value - true or false.

Note: Instead of a condition, there could even be a variable which returns a non-zero value or just a non-zero value.




Working of if tag


The if tag starts with the {% if %} tag and ends with a {% endif %} tag. All the texts between the start and end tag is known as a block. Let us see a code with syntax of an if tag.

{% if var %}
	<p>This is associated with the if tag </p>
{% endif %}






Using Relational Operators with if tag


We could use relational operators such as ==, !=, <, >, <=, >= with the if tag to test a condition expression involving multiple variables.

{% if var1 < var2 %}
	<p>var1 is smaller than var2 </p>
{% endif %}

	
{% if var1 > var2 %}
	<p>var1 is greater than var2 </p>
{% endif %}

	
{% if var1 <= var2 %}
	<p>var1 is less than or equal to var2 </p>
{% endif %}

	
{% if var1 >= var2 %}
	<p>var1 is greater than or equal to var2 </p>
{% endif %}

	
{% if var1 == var2 %}
	<p>var1 is equal to var2 </p>
{% endif %}

	
{% if var1 != var2 %}
	<p>var1 is not equal to var2 </p>
{% endif %}





Using logical operators with if tag


We can use logical operator like and, or, not with the if tag to test a condition involving multiple variables, or, to negate a given variable.

{% if var1>0 and var2<10 %}
	<p>var1 is greater than 0 and if var is less than 10 </p>
{% endif %}

	
{% if var1>0 or var2<10 var2 %}
	<p>var1 is greater than zero or var2 is less than 10 </p>
{% endif %}

	
{% if not var1 %}
	<p>The negation or inverse of var1 is true </p>
{% endif %}





Precedence of operators when used with if tag


The precedence of the operators when used in condional expressions in the if tag from highest to lowest, is as follows:



Django project with if tag


Up next, we are going to explain a situation is which the Django Template System encounters the if 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 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 = { 'var1' : 100,
    'var2' : 200,
    'var3' : 300 }
	
	
    #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 :



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 -



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 -



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 if template tag in HTML Template file to compare the values of two variables var1 and var2 using the relational and logical operators and these variables defined in the view function.




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