Understanding URL Routing in Flask — SitePoint


On this article, we’ll discover URL routing in Flask, discussing its significance in internet growth and offering insights into the way it works.

We’ll delve into numerous features, akin to defining routes, dealing with dynamic URL patterns, supporting totally different HTTP strategies, managing redirects and errors, and following greatest practices for efficient URL routing in Flask.

Desk of Contents
  1. Flask and URL Routing
  2. Fundamental Routing in Flask
  3. Variable Guidelines
  4. URL Constructing
  5. HTTP Strategies
  6. Redirects and Errors
  7. Finest Practices for URL Routing in Flask
  8. Conclusion

Flask and URL Routing

Flask is a well-liked internet framework for Python that allows internet builders to construct internet functions simply and effectively.

That is a complicated article on Flask, so that you’ll must have a fundamental understanding of how Flask works. You’ll be able to rise up to hurry by taking a look at our introduction to Flask. You can even rise up to hurry by wanting over the Flask documentation.

One key options of Flask is its highly effective URL routing system.

URL routing is a elementary idea in internet growth that includes mapping (binding) URLs to particular functionalities or assets inside an internet software. It’s a means of figuring out how incoming requests are dealt with, and by which view features. Routing is about accepting requests and directing them to the suitable view features that may deal with them and generate the specified response.

Fundamental Routing in Flask

Routing in Flask determines how incoming requests are dealt with based mostly on the URL a person has requested. Flask makes use of the route() decorator technique of the Flask software occasion to outline routes after which bind them to applicable view features.

To reveal fundamental routing in Flask, we begin by importing the Flask class from the flask module:

As soon as we have now the Flask class, we will create the appliance occasion and retailer it in a variable referred to as app. The Flask class takes in a __name__ argument, which is a particular Python variable denoting the title of the present module containing the Flask software occasion:

Utilizing the appliance occasion, we have now entry to its numerous strategies and interior designers, which we will use to outline routes, deal with requests, and carry out different duties in our internet software. Nonetheless, for this instance, we’ll have an interest within the route() decorator, a particular technique which, when utilized to a perform in Flask, turns it right into a view perform that may deal with internet requests. It takes in a compulsory URL sample and elective HTTP strategies as its arguments. The route() decorator allows us to affiliate a URL sample with the embellished perform, basically saying that if a person visits the URL outlined within the decorator, the perform will probably be triggered to deal with this request:

def index():
 return "This can be a fundamental flask software"

Within the code snippet above, we have now the route(/) decorator utilized to the index() perform, that means that the perform will deal with requests to the basis URL ’/’. So when a person accesses the URL, Flask will set off the index() perform that may return the string “This can be a fundamental Flask software”, and it is going to be displayed within the browser.

To make sure the appliance runs when this module is invoked with Python within the command line, add the if __name__ verify:

if __name__ == '__main__':

Placing all of the above code snippets into one file offers us the next Flask software:

from flask import Flask

app = Flask(__name__)

def index():
 return "This can be a fundamental flask software"

if __name__ == '__main__':

By utilizing the route() decorator, we will outline routes for various URLs and map them to the suitable view features that may generate the specified response. This enables us to create a structured and arranged internet software with distinct functionalities for various routes.

Variable Guidelines

Within the instance above, we created a easy URL. Flask allows us to create dynamic URLs that may reply to varied eventualities, person enter and particular necessities. By passing variables in URL patterns, builders can design routes that adapt dynamically and ship personalised, participating experiences to customers.

When defining routes in Flask, we will embrace variable placeholders marked by <variable_name> within the URL sample. By default, these variables maintain string values. Nonetheless, if we have to move different varieties of information, Flask gives converters that allow us to specify the kind of information to be captured, like this: <converter:variable_name>.

Under is an instance diagram exhibiting numerous variable URLs. The URLs change relying on the worth a person provides. For instance, we might fetch a distinct product each time by giving a distinct ID, or we might present a distinct creator profile by altering the username on the URL.

An image showing URLs that take in variables

Say we have now a running a blog software and we need to create a URL for a view that exhibits the profile of an creator. We might move the username of the creator like so:

def show_author(username):
 return f"Return the creator profile of {username}"

On this instance, the URL sample is '/authors/<username>', the place <username> is the variable that will probably be changed with the precise username of the creator. Will probably be handed to the show_author() perform as a parameter. Utilizing this worth, we will carry out additional actions, akin to retrieving the creator information from the database and producing a response based mostly on the data we get from the database.

We are able to additionally move a couple of variable in a URL:

def show_post(post_id, slug):
 return f"Submit {post_id} - Slug: {slug}"

On this instance, we have now a URL sample that has a couple of variable rule. It additionally incorporates a converter for the post_id within the type of <int:post_id>, indicating that an integer is predicted to the variable. We even have <slug>, which is able to seize a slug string for this variable.

So when a request is made to this URL, Flask will seize the values specified within the URL and move them as arguments to the show_post() perform for additional processing. For instance, if a request is made to /posts/456/flask-intro, Flask will seize post_id=456 and slug=flask-intro, and these values will probably be handed as arguments to the show_post() perform. Inside the perform, we will carry out numerous operations, akin to retrieving the corresponding submit from a database based mostly on the post_id and slug values. Within the instance code, we merely return a string response that features the captured values:

return f"Submit {post_id} - Slug: {slug}"

This instance exhibits how we will use variable guidelines and converters in Flask to create dynamic URLs that seize various kinds of information — such integers and strings — and course of them inside view features. This enables us to construct apps that reply to particular person enter and ship customized content material.

URL Constructing

As soon as we outline our URL patterns and map them to view features, we will use them wherever else in our code or within the templates, as an alternative of exhausting coding the URLs Flask gives the url_for() perform. The perform will mechanically construct a URL relying on the arguments we offer to it. It takes the title view perform as the primary required argument and any variety of elective key phrase arguments, every equivalent to a variable a part of the URL rule.

Constructing URLs utilizing the url_for() perform has a number of advantages. It’s extra descriptive in our code if we have now one thing like url_for(‘login’) as an alternative of /login. We’ll additionally be capable to change our URLs in a single go as an alternative of needing to recollect to manually change all hard-coded URLs. This perform additionally handles escaping of particular characters transparently and mechanically. The generated URLs will probably be absolute paths, eradicating the issue of relative paths in browsers; regardless of the construction of our software, the url_for() perform will deal with that for us correctly.

Let’s use the url_for() perform to generate URLs for the view features we’ve already seen above:

from flask import Flask, url_for

app = Flask(__name__)

def index():
 return "This can be a fundamental flask software"

def show_author(username):
  return f"Return the creator profile of {username}"

def show_post(post_id):
 return f"Exhibiting submit with ID: {post_id}"

if __name__ == '__main__':
 with app.test_request_context():
   home_url = url_for('index')
   profile_url = url_for('show_author', username='antony')
   post_url = url_for('show_post', post_id=456, slug='flask-intro' )

   print("Generated URLs:")
   print("Residence URL:", home_url)
   print("Writer profile URL:", profile_url)
   print("Submit URL:", post_url)

This instance demonstrates the utilization of the url_for() perform to generate URLs. So as to use the perform, we import it from the flask module. It defines three routes: "https://www.sitepoint.com/", '/authors/<username>', and '/submit/<int:post_id>/<slug>'.

Inside the if __name__ == '__main__': block, a take a look at request context is created utilizing app.test_request_context() to permit entry to the url_for() perform. It tells Flask to behave as if it’s dealing with a request even whereas we use the shell.

We then retailer the generated URLs in variables (home_url, profile_url, and post_url) after which print them, giving an output just like the one proven beneath:

Generated URLs:
Residence URL: /
Writer profile URL: /authors/antony
Submit URL: /submit/456/flask-intro

We are able to additionally use the url_for() in templates. Let’s replace the Flask software to render templates and see how we will transfer from one template to the opposite, by producing URLs utilizing the url_for() perform:

from flask import Flask, render_template

app = Flask(__name__)

def index():
   return render_template("index.html")

def show_author(username):
   return render_template('profile.html', username=username)

if __name__ == '__main__':

Create a templates listing within the root of the undertaking and create the next two information.


<!DOCTYPE html>
 <title>Residence Web page</title>
 <h1>Welcome to the house web page!</h1>
 <a href="{{ url_for('show_author', username='Antony') }}">Go to Antony's profile</a>


<!DOCTYPE html>
 <title>Consumer Profile</title>
 <h1>Welcome, {{ username }}!</h1>
 <a href="{{ url_for('index') }}">Return to residence web page</a>

Within the index.html template, we use the url_for() perform to generate the URL for the 'profile' endpoint and move the username argument as 'Antony'. This generates a hyperlink to Antony’s profile web page. Equally, within the profile.html template, we generate a hyperlink to the house web page utilizing url_for('index').

When the templates are rendered, Flask will exchange {{ url_for(...) }} with the corresponding generated URLs. This enables for dynamic URL technology throughout the templates, making it simple to create hyperlinks to different routes or move arguments to these routes.

Utilizing url_for() in templates helps make sure that the generated URLs are appropriate and maintainable, even when the routes change sooner or later. It gives a handy option to create dynamic hyperlinks that adapt to the present state of our Flask software.

Total, we will see that, by utilizing the url_for() perform, we will dynamically generate URLs in Flask based mostly on the route endpoints (the title of the view perform) and different elective arguments. This gives a versatile and dependable option to create URLs that adapt to the particular functionalities of our internet app.

HTTP Strategies

Internet functions use totally different HTTP strategies when accessing URLs, and functions constructed utilizing the Flask framework are not any exception. Flask helps numerous HTTP strategies akin to GET, POST, PUT, DELETE and extra. These strategies outline the actions we will perform on the assets which might be obtainable when accessing the varied URLs we’ve outlined in our software. Utilizing the totally different HTTP strategies, we will deal with various kinds of requests and carry out associated operations in our Flask software.

In Flask, we will outline the HTTP strategies {that a} route can settle for utilizing the strategies parameter of the route decorator. After we specify the strategies a route accepts, Flask ensures that the route is barely accessible for these specified strategies.

Right here’s an instance:

from flask import Flask, request

app = Flask(__name__)

@app.route("https://www.sitepoint.com/", strategies=['GET'])
def index():
 return "That is the house web page"

@app.route('/authors', strategies=['GET', 'POST'])
def authors():
 if request.technique == 'GET':
    return "Get all authors"
 elif request.technique == 'POST':
    return "Create a brand new creator"

@app.route('/authors/<int:author_id>', strategies=['GET', 'PUT', 'DELETE'])
def creator(author_id):
 if request.technique == 'GET':
    return f"Get creator with ID: {author_id}"
 elif request.technique == 'PUT':
    return f"Replace creator with ID: {author_id}"
 elif request.technique == 'DELETE':
    return f"Delete person with ID: {author_id}"

if __name__ == '__main__':

This up to date code demonstrates the utilization of HTTP strategies in Flask. It defines three routes: "https://www.sitepoint.com/",'/authors', and '/authors/<int:author_id>'. Every route has particular HTTP strategies related to it.

The '/‘ route solely permits GET requests, and the index() perform handles GET requests to this route. It returns the string That is the house web page when accessed by way of a GET request. We are able to omit the GET technique parameter, since GET is the default for all strategies except explicitly said.

The '/authors' route permits each GET and POST requests, and the authors() perform handles these requests. If the request technique is GET, it returns the string Get all authors. If the request technique is POST, it performs the mandatory actions to create a brand new creator and returns the string Create a brand new creator.

The '/authors/<int:author_id>' route permits GET, PUT, and DELETE requests, and the creator() perform handles these requests. If the request technique is GET, it retrieves the creator with the required ID and returns a response string. If the request technique is PUT, it updates the creator with the required ID and returns a response string. If the request technique is DELETE, it deletes the creator with the required ID and returns a response string.

By defining routes with particular HTTP strategies, we will create a RESTful API or internet software that handles various kinds of requests and performs the suitable operations on the related assets.

Redirects and Errors

As we undergo the idea of routing in Flask, we additionally want to grasp easy methods to deal with errors. Customers will present improper URLs or incomplete info, and this may result in errors occurring in our software. So for our software to be full, we’ll must deal with errors gracefully — both by offering informative messages, or by redirecting customers. Flask gives the redirect and abort features.

The redirect perform is used to take the person to a brand new URL. It may be utilized in eventualities akin to profitable kind submission, authentication, or after we need to information a person to a distinct part of the appliance. It takes within the URL as an argument or the route title. It returns a standing code 302 by default, however we will outline our personal customized standing codes.

To deal with errors, the abort perform is offered by Flask. It’s used to abort the processing of a request and return a HTTP error response. This can permit us to deal with distinctive circumstances or errors in our software and reply with the suitable HTTP standing code and error web page. Utilizing this perform, we will deal with numerous errors in Flask, akin to unauthorized entry, invalid requests and other forms of errors. We are able to select the suitable HTTP standing code for the error, guaranteeing the shopper will get details about what went improper.

Right here’s an instance that exhibits easy methods to use the 2 features:

from flask import Flask, redirect, render_template, request, abort

app = Flask(__name__)

def residence():
 return render_template('residence.html')

@app.route('/login', strategies=['GET', 'POST'])
def login():
 if request.technique == 'POST':
  username = request.kind['username']
  password = request.kind['password']

  if username == 'admin' and password == 'password':
           return redirect('/dashboard')

  return render_template('login.html')

def dashboard():
 return render_template('dashboard.html')

def unauthorized_error(error):
 return render_template('unauthorized.html'), 401

if __name__ == '__main__':

On this instance, the /login route handles each GET and POST requests. When a POST request is acquired, it performs authentication with the offered credentials. If the credentials are legitimate, the person is redirected to the /dashboard route utilizing the redirect perform.

Nonetheless, if the authentication fails (akin to when the person gives the improper login info), the request is aborted with a 401 Unauthorized standing code utilizing the abort perform. The person is then directed to an error web page specified within the unauthorized_error error handler, which renders the unauthorized.html template and returns a 401 standing code.

By using the redirect and abort features collectively, we will successfully deal with authentication failures and redirect customers to applicable pages or show related error messages, guaranteeing a safe and user-friendly login expertise.

Finest Practices for URL Routing in Flask

It’s necessary for our software to observe one of the best practices when creating URLs. Listed below are among the most necessary to observe:

  • Set up URLs and make them simple to learn. Group the associated routes. This can make it simpler to navigate and handle our codebase.
  • Make use of variables. Utilizing variables in URL patterns will allow our software to deal with dynamic requests from customers. To do that, we will make the most of guidelines akin to <variable_name>. We are able to couple variables with converters to be able to deal with totally different varieties of information in our URLs.
  • Clear error messages. Be certain that, when dealing with errors in routes, we give clear and informative error messages to customers. This can go a good distance in direction of serving to customers perceive why the errors occurred and to taking the suitable actions.
  • Make the most of the url_for perform. Constructing URLs utilizing the url_for perform ensures that our URLs are mechanically generated and correctly structured and constant all through the appliance, eliminating the necessity to exhausting code them.

By following these greatest practices, we will create well-organized and maintainable URL routing in our Flask functions. This results in cleaner code, improved readability, higher error dealing with, and extra versatile and structured URLs.


In conclusion, understanding URL routing in Flask is important for constructing person pleasant and highly effective internet functions. By defining routes and mapping them to view features, this permits builders to deal with incoming requests and generate applicable responses. When coupled with variables, we’re capable of construct extra dynamic URL patterns, making the appliance extra versatile and adaptable.

Defining the type of HTTP strategies a route accepts allows an software to simply accept various kinds of request relying on the strategy outlined for a route.

Flask additionally gives options for dealing with errors and redirects. The redirect perform allows us to redirect customers to totally different URLs, whereas the abort perform helps in dealing with errors and responding with applicable HTTP standing codes.

A number of the greatest practices for URL routing in Flask embrace conserving routes organized and simple to learn, utilizing variables for dynamic URL patterns, offering clear messages for widespread errors, and correctly structuring and producing URLs utilizing the url_for perform.

By understanding URL routing ideas in Flask, and following these greatest practices, builders can create environment friendly, versatile and user-friendly internet functions.


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