Set url for Tornado handlers

Set url for Tornado handlers

To set url for tornado handlers we can pass list of tuples (url regex, handler) into application initialisation:

application = tornado.web.Application([
    (r"/", MainHandler),
    (r"/some/path/page/(?P<pk>[0-9]+)$", PageHandler),
])

But it is more convenient to use wrapper tornado.web.url, that allows to assign meaningful names for paths (similar to django url).

Nevertheless, in a couple of production projects that i had to work with, this wrapper wasn't used. Also in some tornado examples from documentation (one, two, three) simple tuples are used, that can be confusing. So i think that it is worth to mention the advantages, that gives url wrapper.

So, what disadvantages we'll face in case of tuples, i.e. without using a url.

Without url()

To represent needed path in code or in template, we have to manually enter the string.

Example.

app.py:

import tornado.ioloop
import tornado.web

class MainHandler(tornado.web.RequestHandler):
    def get(self):
        self.render("home.html", title="My title", pages=[1, 2, 3])

class PageHandler(tornado.web.RequestHandler):
    def get(self, page_n):
        email_text = "Please visit this page: '/some/path/page/{page_n}/'".format(
            page_n=1)
        send_email('some@person.com', email_text)
        self.render("page.html", title="Page", page_n=page_n)

application = tornado.web.Application([
    (r"/", MainHandler),
    (r"/some/path/page/(?P<page_n>[0-9]+)/$", PageHandler),
])

if __name__ == "__main__":
    application.listen(8888)
    tornado.ioloop.IOLoop.instance().start()

home.htm:

<html>
   <head>
      <title>{{ title }}</title>
   </head>
   <body>
     <div>View pages:</div>
     <ul>
       {% for page_n in pages %}
         <li><a href="/some/path/page/{{ page_n }}/">{{ page_n }}</a></li>
       {% end %}
     </ul>
   </body>
 </html>

page.html:

<html>
   <head>
      <title>{{ title }}</title>
   </head>
   <body>
     <div>You are viewing page #{{ page_n }}</div>
     <div>Back to <a href="/">Home<a></div>
   </body>
 </html>

As we can see, even in this simple code we repeat path /some/path/page/ three times. What if we'll need to change this string a little? We'll have to make an autocorrect, that is uncomfortably and can lead to errors. Furthermore, some paths can be cumbersome and decrease code readability.

With  url()

Same example, but with url:

app.py:

import tornado.ioloop
import tornado.web
from tornado.web import url


class MainHandler(tornado.web.RequestHandler):
    def get(self):
        self.render("home.html", title="My title", pages=[1, 2, 3])


class PageHandler(tornado.web.RequestHandler):
    def get(self, page_n):
        email_text = "Please visit this page: '{url}'".format(
            url=self.reverse_url('page', 1))
        send_email('some@person.com', email_text)
        self.render("page.html", title="Page", page_n=page_n)


application = tornado.web.Application([
    url(r"/", MainHandler, name="home"),
    url(r"/some/path/page/(?P<page_n>[0-9]+)/$", PageHandler, name="page"),
])

if __name__ == "__main__":
    application.listen(8888)
    tornado.ioloop.IOLoop.instance().start()

home.html:

<html>
   <head>
      <title>{{ title }}</title>
   </head>
   <body>
     <div>View pages:</div>
     <ul>
       {% for page_n in pages %}
         <li><a href="{{reverse_url('page', page_n)}}">{{ page_n }}</a></li>
       {% end %}
     </ul>
   </body>
 </html>

page.html:

<html>
   <head>
      <title>{{ title }}</title>
   </head>
   <body>
     <div>You are viewing page #{{ page_n }}</div>
     <div>Back to <a href="{{reverse_url('home')}}">Home<a></div>
   </body>
 </html>

Paths now have meaningful names that are used in url representation by method reverse_url. If it is needed to change some path, we'll do it in one single place. Much more convenient!

Published: Dec. 15, 2014
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