Fix Cors Issue With Flutter for the web and Django

Flutter and Django are two powerful frameworks that can be used to build web applications. However, when you try to make a request from a Flutter web app to a Django backend, you might run into a CORS (Cross-Origin Resource Sharing) issue. In this article, I will explain what is CORS and how to fix it when using Flutter for the web and Django.

What is CORS?

CORS is a security feature implemented by web browsers to prevent malicious websites from making requests to other websites on behalf of the user. When a web application makes a request to a different domain, the browser checks if the server allows the request by looking at the Access-Control-Allow-Origin header. If the server does not allow the request, the browser will block it and show an error message.

Cross-Origin Resource Sharing (CORS) is not a mandatory feature, your backend may or may not implement it. However, if you are building a modern web application, you will likely need to deal with CORS at some point.

Fixing CORS issue with Django and Flutter

When you first build your Flutter web app and try to make a request to your Django backend, you might see an error message like this in the browser console:

Access to XMLHttpRequest at 'http://localhost:8000/api/posts/' from origin 'http://localhost:5000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

This error is pretty self-explanatory. The Django server is not allowing requests from the Flutter web app because the Access-Control-Allow-Origin header is missing.

The reason behind this is that by default, Django does not add the Access-Control-Allow-Origin header to the response. To fix this, you need to add the corsheaders package to your Django project.

Step 1: Install the corsheaders package

First, you need to install the corsheaders package in your Django project. You can do this using pip:

pip install django-cors-headers

Step 2: Add corsheaders to your Django project

Next, you need to add corsheaders to the INSTALLED_APPS in your Django project’s settings.py file:

INSTALLED_APPS = [
    ...
    'corsheaders',
    ...
]

Step 3: Configure corsheaders

Finally, you need to configure corsheaders in your Django project’s settings.py file. Add the following lines to the end of the file:

CORS_ALLOWED_ORIGINS = [
    "http://localhost:5000",
]

Previously this setting was called CORS_ORIGIN_WHITELIST, which still works as an alias, with the new name taking precedence.

Note : While the settings CORS_ORIGIN_ALLOW_ALL set to True is the easiest way to allow all origins, it is not recommended for production as it can open your server to security vulnerabilities.

Step 4: Restart your Django server

After making these changes, restart your Django server. Now, when you make a request from your Flutter web app to your Django backend, you should no longer see the CORS error message in the browser console.

Conclusion

This error was a common issue when I was building a Flutter web app that communicates with a Django backend. By understanding what CORS is and how to fix it, you can ensure that your web application works as expected.