Troubleshooting Inefficient Template Rendering in Django

Templates in Django play the key role of separating the app’s logic from the application's presentation layer. However, as your application grows so does the complexity of the templates that you use, which might lead to inefficient template rendering. In this article, you’ll learn about:

• Introduction to Django templates
• What is templating in Django
• The importance of templating in Django
• Causes of inefficient template rendering in Django
• Troubleshooting inefficient template rendering in Django

Introduction to Django template

A Django template is a static markup file that defines the structure of a webpage, and the syntax of the HTML used to render dynamic views on the browser. Imagine mixing your code's logic with the markup(HTML) to render the user views. This would make your code hard to maintain and hard for the backend and front-end teams to collaborate to fix bugs or add new code features. That is the problem templating solves in Django, and it allows developers to separate the HTML to render the view from the Python code.

Templating in Django

Django views store data from the database, and templates provide a framework to display this data. Think of views as data holders and templates as the presentation canvas waiting to be filled. Templating involves replacing the placeholders in the templates with data fetched from the views. This could involve looping through an array response and displaying the data in a list or table on the UI.

The importance of templating in Django

Some programming languages such as ASP, the logic of the app and the HTML markup code are all contained in the same file which makes it easier for developers to study a single-file page because you can see the code and the markup in one place. However, this becomes hard to maintain and debug as the application gets bigger and more complex. This isn’t a good way to develop applications that scale and are maintainable. The following are some of the reasons you should use templates to separate your Django code from the browser views:

Reusability
In most applications, there are standard UI views that appear in multiple parts of the website such as the header, navigation bar, and footer. To add them to all the web pages of your app, you will have to copy and paste the same code in various views in the codebase. Templates make it possible to create one standard template that can be included in all the views that need a navigation bar for example. As your app grows, you can include the templates in your new views without much hassle.

Maintainability
It’s easier to maintain an application whose logic, models, and views are written separately. Even new developers can quickly find the templates they need to add new logic to or improve the logic without wasting a lot of time finding the right file.

Easier collaboration
Separating logic and view code makes it so that front-end developers and designers can work on the templates while back-end developers can work on the Django logic code. This separation of the codebase encourages easier collaboration across the team, which shortens the project's delivery time.

Easier onboarding
It’s simpler for new developers to get acquitted with your codebase and also make their first contribution to a Django app that uses templates since there’s a shorter learning curve.

Causes of inefficient template rendering

The Django templating system has a few advantages but it also comes with its own drawbacks. The process of rendering HTML on the browser from templates becomes inefficient when it takes longer than necessary. Listed below are some of the factors that may slow down template rendering:

Slow rendering when querying large datasets from the database
Template load time is directly related to how much data is being loaded from the database to the UI. The more the data e.g., if you’re querying for 1000 objects to populate a listview, the longer the load time and vice versa.

Multiple templates
In certain scenarios, it becomes necessary to present a single template in various formats. For instance, many banking applications adopt a distinct theme for festive seasons while relying on regular templates for the rest of the year. Additionally, there are instances where a different template is required for mobile devices. At times you also need to use a different template for mobile devices. The more templates there are to load a single webpage, the more complex the logic to render the webpage gets, which in turn increases the app’s load time.

Complex template logic
The template context becomes accessible while the template is being rendered. Django can handle simple logic such as performing conditionals but when complex logic such as nested loops are involved, the app’s load time exponentially increases. For example, using the {% for %} tag to loop through a large dataset may cause a slowdown.

Inefficient use of template syntax
Adjusting values in template variables can noticeably slow down template rendering and thus should be avoided for faster performance. Django templates provide support for working with lists and tuples, and you can manipulate them in the templates using list comprehension, slicing, and other methods. However, using these features incorrectly can lead to inefficient rendering.
For example, in this case, you might add some Python code to your template:

{% for item in items %}
  <p>{{ item.name }}</p>
  <p>{{ item.description }}</p>
  <p>{{ item.price }}</p>
  <p>{{ item.calculate_discount(item.price) }}</p>
{% endfor %}

Incorrect use of template tags and filters
Template tags and filters are built-in Django features that enable you to manipulate the data that is in your templates. Filters modify how data is displayed for example, a dictsortreversed filter will sort and reverse a dictionary in your template. Tags on the other hand allow you to define some logic in your template such as a comment and everything between {% comment %} and {% endcomment %} tags gets ignored during template rendering. However, if you use these two features incorrectly, it either leads to errors or unexpected behavior in your apps. For example, the code snippet below shows incorrect logic used to check if a value is divisible by 3 since it passes on a string which will throw an error:

{% for item in items %}
 <p>{{ item.name }}</p>
 <p>{{ item.description }}</p>
 <p>{{ item.price }}</p>
 <p>{{ item.price|divisibleby:"3" }}</p>
{% endfor %}

Failure to cache
Caching involves storing rendered data and using it again in later requests. Enabling template caching in Django is commonly recommended as it speeds up the parsing and rendering of templates. Failure to cache database queries, for example, will exponentially increase the overhead of rendering them as the application grows making the app close to unusable due to slow load times. The screenshot below shows the performance of an application in Scout APM before implementing template caching. You’ll see that close to 80% of the page’s load time is used to render the view highlighting the massive impact of not caching. This points out the need to implement caching to address this considerable portion of the load time.

Troubleshooting inefficient template rendering in Django

This section delves into the art of troubleshooting inefficient template rendering in Django to make them more performant.

Check your template syntax
You should always check your template syntax to identify complex syntax that causes inefficient rendering. For example, if you're calling a method directly in your template, this can slow down rendering. Instead, you should calculate any necessary values in your view and then pass them to your template as shown in the template below:

# In your view
items = Item.objects.all()
for item in items:
  item.discount = item.calculate_discount(item.price)

# In your template
{% for item in items %}
  <p>{{ item.name }}</p>
  <p>{{ item.description }}</p>
  <p>{{ item.price }}</p>
  <p>{{ item.discount }}</p>
{% endfor %}

Use the Django debug toolbar
The Django debug toolbar is a tool that comes with a set of panels that shows the debug information of the requests/responses of a given page. It shows details such as how long specific requests take before they return a response. While the Chrome debugs toolbar is used to debug the front end of a Django app, the Django debug toolbar debugs the server-side requests of the Django app e.g. database queries.

Caching using the cached template loader
Django apps compile and load templates every time they are being rendered. If you’re loading multiple templates such as a header, footer, navigation panel, etc., on a single webpage, the page load time will increase as the page features increase. Caching will help load your pages quickly because once the template is requested for the first time, the cached template loader will store the template’s content in memory. Each subsequent request after that, will be served directly from the cache thus improving the app’s performance. Once you implement template caching in your app, you should see the load time significant drop in your app’s load time from Scout APM’s monitoring dashboard. After caching, the time it takes to render the page goes down from 137ms to 47 ms on the Scout APM dashboard:

Correct usage of Django tags and filters
These two features if misused, will lead to errors or unexpected behavior in your apps. For illustration purposes, consider the following code snippet that shows logic used to check that a value is divisible by 3. If a string value is passed to the divisibleby filter, it will return an error.

{% for item in items %}
 <p>{{ item.name }}</p>
 <p>{{ item.description }}</p>
 <p>{{ item.price }}</p>
 <p>{{ item.price|divisibleby:"3" }}</p>
{% endfor %}

Therefore, you need to ensure that the correct data types are used in your tags and filters diminishing the likelihood of encountering issues or undesired outcomes in your application.

{% if item.price|divisibleby:3 %}
 <p>Price is divisible by 3</p>
{% endif %}

Conclusion

In conclusion, mastering Django template rendering is not merely a matter of syntax and structure; it is the essential concept that separates a robust, scalable application from one with template inefficiencies. So, as you embark on your journey to tap the full potential of Django templates, remember: the devil is in the details and efficient templates are the key to unlocking a performant, scalable application.