4. Routing

Modern web applications have beautiful URLs. This helps people remember the URLs, which is especially handy for applications that are used from mobile devices with slower network connections. If the user can directly go to the desired page without having to hit the index page it is more likely they will like the page and come back next time.

As you have seen above, the route() decorator is used to bind a function to a URL. Here are some basic examples:

def index():
    return 'Index Page'

def hello():
    return 'Hello, World'

But there is more to it! You can make certain parts of the URL dynamic and attach multiple rules to a function.

4.1. Variable Rules

To add variable parts to a URL you can mark these special sections as <variable_name>. Such a part is then passed as a keyword argument to your function. Optionally a converter can be used by specifying a rule with <converter:variable_name>. Here are some nice examples:

def show_user_profile(username):
    # show the user profile for that user
    return 'User %s' % username

def show_post(post_id):
    # show the post with the given id, the id is an integer
    return 'Post %d' % post_id

The following converters exist:

string accepts any text without a slash (the default)
int accepts integers
float like int but for floating point values
path like the default but also accepts slashes
any matches one of the items provided
uuid accepts UUID strings


Unique URLs / Redirection Behavior

Flask’s URL rules are based on Werkzeug’s routing module. The idea behind that module is to ensure beautiful and unique URLs based on precedents laid down by Apache and earlier HTTP servers.

Take these two rules:

def projects():
    return 'The project page'

def about():
    return 'The about page'

Though they look rather similar, they differ in their use of the trailing slash in the URL definition. In the first case, the canonical URL for the projects endpoint has a trailing slash. In that sense, it is similar to a folder on a filesystem. Accessing it without a trailing slash will cause Flask to redirect to the canonical URL with the trailing slash.

In the second case, however, the URL is defined without a trailing slash, rather like the pathname of a file on UNIX-like systems. Accessing the URL with a trailing slash will produce a 404 “Not Found” error.

This behavior allows relative URLs to continue working even if the trailing slash is omitted, consistent with how Apache and other servers work. Also, the URLs will stay unique, which helps search engines avoid indexing the same page twice.

4.2. URL Building

If it can match URLs, can Flask also generate them? Of course it can. To build a URL to a specific function you can use the url_for() function. It accepts the name of the function as first argument and a number of keyword arguments, each corresponding to the variable part of the URL rule. Unknown variable parts are appended to the URL as query parameters. Here are some examples:

>>> from flask import Flask, url_for
>>> app = Flask(__name__)
>>> @app.route('/')
... def index(): pass
>>> @app.route('/login')
... def login(): pass
>>> @app.route('/user/<username>')
... def profile(username): pass
>>> with app.test_request_context():
...  print url_for('index')
...  print url_for('login')
...  print url_for('login', next='/')
...  print url_for('profile', username='John Doe')

(This also uses the test_request_context() method, explained below. It tells Flask to behave as though it is handling a request, even though we are interacting with it through a Python shell. Have a look at the explanation below. Context Locals).

Why would you want to build URLs using the URL reversing function url_for() instead of hard-coding them into your templates? There are three good reasons for this:

  1. Reversing is often more descriptive than hard-coding the URLs. More importantly, it allows you to change URLs in one go, without having to remember to change URLs all over the place.
  2. URL building will handle escaping of special characters and Unicode data transparently for you, so you don’t have to deal with them.
  3. If your application is placed outside the URL root - say, in /myapplication instead of / - url_for() will handle that properly for you.

4.3. HTTP Methods

HTTP (the protocol web applications are speaking) knows different methods for accessing URLs. By default, a route only answers to GET requests, but that can be changed by providing the methods argument to the route() decorator. Here are some examples:

from flask import request

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':

If GET is present, HEAD will be added automatically for you. You don’t have to deal with that. It will also make sure that HEAD requests are handled as the HTTP RFC (the document describing the HTTP protocol) demands, so you can completely ignore that part of the HTTP specification. Likewise, as of Flask 0.6, OPTIONS is implemented for you automatically as well.

You have no idea what an HTTP method is? Worry not, here is a quick introduction to HTTP methods and why they matter:

The HTTP method (also often called “the verb”) tells the server what the client wants to do with the requested page. The following methods are very common:

The browser tells the server to just get the information stored on that page and send it. This is probably the most common method.
The browser tells the server to get the information, but it is only interested in the headers, not the content of the page. An application is supposed to handle that as if a GET request was received but to not deliver the actual content. In Flask you don’t have to deal with that at all, the underlying Werkzeug library handles that for you.
The browser tells the server that it wants to post some new information to that URL and that the server must ensure the data is stored and only stored once. This is how HTML forms usually transmit data to the server.
Similar to POST but the server might trigger the store procedure multiple times by overwriting the old values more than once. Now you might be asking why this is useful, but there are some good reasons to do it this way. Consider that the connection is lost during transmission: in this situation a system between the browser and the server might receive the request safely a second time without breaking things. With POST that would not be possible because it must only be triggered once.
Remove the information at the given location.
Provides a quick way for a client to figure out which methods are supported by this URL. Starting with Flask 0.6, this is implemented for you automatically.

Now the interesting part is that in HTML4 and XHTML1, the only methods a form can submit to the server are GET and POST. But with JavaScript and future HTML standards you can use the other methods as well. Furthermore HTTP has become quite popular lately and browsers are no longer the only clients that are using HTTP. For instance, many revision control systems use it.