Flask – Variable Rules


It is possible to build a URL dynamically, by adding variable parts to the rule parameter. This variable part is marked as <variable-name>. It is passed as a keyword argument to the function with which the rule is associated.

In the following example, the rule parameter of route() decorator contains <name> variable part attached to URL ‘/hello’. Hence, if the http://localhost:5000/hello/TutorialsPoint is entered as a URL in the browser, ‘TutorialPoint’ will be supplied to hello() function as argument.

from flask import Flask
app = Flask(__name__)

def hello_name(name):
   return 'Hello %s!' % name

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

Save the above script as hello.py and run it from Python shell. Next, open the browser and enter URL http://localhost:5000/hello/TutorialsPoint.

The following output will be displayed in the browser.

Hello TutorialsPoint!

In addition to the default string variable part, rules can be constructed using the following converters −

Sr.No Converters & Description


accepts integer



For floating point value



accepts slashes used as directory separator character

In the following code, all these constructors are used.

from flask import Flask
app = Flask(__name__)

def show_blog(postID):
   return 'Blog Number %d' % postID

def revision(revNo):
   return 'Revision Number %f' % revNo

if __name__ == '__main__':

Run the above code from Python Shell. Visit the URL http://localhost:5000/blog/11 in the browser.

The given number is used as argument to the show_blog() function. The browser displays the following output −

Blog Number 11

Enter this URL in the browser − http://localhost:5000/rev/1.1

The revision() function takes up the floating point number as argument. The following result appears in the browser window −

Revision Number 1.100000

The URL rules of Flask are based on Werkzeug’s routing module. This ensures that the URLs formed are unique and based on precedents laid down by Apache.

Consider the rules defined in the following script −

from flask import Flask
app = Flask(__name__)

def hello_flask():
   return 'Hello Flask'

def hello_python():
   return 'Hello Python'

if __name__ == '__main__':

Both the rules appear similar but in the second rule, trailing slash (/) is used. As a result, it becomes a canonical URL. Hence, using /python or /python/ returns the same output. However, in case of the first rule, /flask/ URL results in 404 Not Found page.