Flask – Redirect & Errors

Flask class has a redirect() function. When called, it returns a response object and redirects the user to another target location with specified status code.
Prototype of redirect() function is as below −

Flask.redirect(location, statuscode, response)

In the above function −

• location parameter is the URL where response should be redirected.
• statuscode sent to browser’s header, defaults to 302.
• response parameter is used to instantiate response.

The following status codes are standardized −

• HTTP_300_MULTIPLE_CHOICES
• HTTP_301_MOVED_PERMANENTLY
• HTTP_302_FOUND
• HTTP_303_SEE_OTHER
• HTTP_304_NOT_MODIFIED
• HTTP_305_USE_PROXY
• HTTP_306_RESERVED
• HTTP_307_TEMPORARY_REDIRECT

The default status code is 302, which is for ‘found’.
In the following example, the redirect() function is used to display the login page again when a login attempt fails.

from flask import Flask, redirect, url_for, render_template, request
# Initialize the Flask application
app = Flask(__name__)

@app.route('/')
def index():
   return render_template('log_in.html')

@app.route('/login',methods = ['POST', 'GET'])
def login():
   if request.method == 'POST' and
   request.form['username'] == 'admin' :
   return redirect(url_for('success'))
   return redirect(url_for('index'))

@app.route('/success')
def success():
   return 'logged in successfully'
 
if __name__ == '__main__':
   app.run(debug = True)

Flask class has abort() function with an error code.

Flask.abort(code)

The Code parameter takes one of following values −

• 400 − for Bad Request
• 401 − for Unauthenticated
• 403 − for Forbidden
• 404 − for Not Found
• 406 − for Not Acceptable
• 415 − for Unsupported Media Type
• 429 − Too Many Requests

Let us make a slight change in the login() function in the above code. Instead of re-displaying the login page, if ‘Unauthourized’ page is to be displayed, replace it with call to abort(401).

from flask import Flask, redirect, url_for, render_template, request, abort
app = Flask(__name__)

@app.route('/')
def index():
   return render_template('log_in.html')

@app.route('/login',methods = ['POST', 'GET'])
def login():
   if request.method == 'POST':
      if request.form['username'] == 'admin' :
         return redirect(url_for('success'))
      else:
         abort(401)
   else:
      return redirect(url_for('index'))

@app.route('/success')
def success():
   return 'logged in successfully'

if __name__ == '__main__':
   app.run(debug = True)