Python Flask - Redirect and Errors

A redirect is used to send a user from one URL to another. When a redirect occurs, server responds with an HTTP status code that instructs the browser to load a different location. Redirects are commonly used after form submissions, authentication checks or when a resource has moved.

Syntax

flask.redirect(location, code=302)

Parameters:

• location (str) URL where the user should be redirected.
• code (int) HTTP status code indicating the type of redirect. If no code is specified, Flask uses 302 (temporary redirect) by default.

Return: a response object that redirects the user to the specified URL with the given HTTP status code.

HTTP Status Codes

HTTP status codes are three-digit numbers returned by the server to indicate how the request should be handled. The following codes are commonly associated with redirects:

Importing redirect Function

To perform URL redirection, we need to import the redirect function from the Flask module:

from flask import redirect

The redirect() function returns a response that instructs the browser to navigate to a different URL.

Example: Redirecting Based on Login Input

In this example, we create a Flask application that redirects users based on the username entered in a login form.

• If the username is admin, the user is redirected to the success page.
• Otherwise, the user is redirected back to the login page.

app.py

HTML Template (login.html)

Create a templates folder in the project directory and add the following file inside templates/login.html

Run the Flask application using:

python app.py

Output

Case 1: If the Username is admin and the method is POST then it will redirect to the success URL and display logged in successfully.

Case 2: If the username is anything other than admin, the application redirects back to the login page, allowing the user to enter the username again.

url_for() Function

url_for() function is used to dynamically generate URLs for a specific function. It takes the function name as the first argument and builds the corresponding URL. This is useful when performing redirects because it avoids hardcoding URLs and makes the application easier to maintain.

Example: Here, the user(name) route checks whether the provided name is admin. If it matches, the request is redirected to the admin page. Otherwise, it redirects to the guest page.

Output: When the URL is /user/admin, the application redirects to the admin page and displays:

When the URL is /user/Ayush, the application redirects to the guest page and displays:

The url_for() function ensures that URLs are generated dynamically, which helps keep routing flexible and prevents issues if route paths change later.

Flasks Errors

abort() function is used to stop request processing and return an HTTP error response. It is commonly used when a requested resource does not exist, request is invalid or access is not allowed.

Syntax

abort(code, message=None)

Parameters:

• code (int) HTTP status code that represents the type of error.
• message (str, optional) custom error message that can be returned with the response.

When abort() is called, Flask immediately stops the current request and returns the specified error code to the client.

HTTP Error Codes

The following HTTP status codes are commonly used when handling errors in Flask applications:

Examples

Example 1: In this example, we check the username provided in the URL using Error Code 400. If username starts with a number, application raises an error using the abort() function. Otherwise, it returns a success message.

Output

Case 1: If the username does not start with a number, the page displays:

Case 2: If the username starts with a number, Flask raises a 400 Bad Request error.

Example 2: In this example, error code in abort() is changed to 403. If the username starts with a number, the application raises a Forbidden error instead.

Output: If the username starts with a number, Flask returns a 403 Forbidden error. Otherwise, page displays "Good Username".