Morepath 0.7: new inter-app linking

I've just released Morepath 0.7!

What is Morepath? Morepath is a Python web framework. It tries to be especially good at implementing modern, RESTful backends. It is very good at creating hyperlinks. It is easy to use, but still lets you write flexible, maintainable and reusable code. Morepath is very extensively documented.

So what's new in Morepath 0.7? The CHANGES doc as usual has the details, but I'll give an overview here.

New features for JSON handling

Morepath 0.7 introduces new ways to deal with JSON. There are two new directives, dump_json and load_json. By using these you can teach Morepath how to automatically convert incoming JSON to a Python object, and outgoing Python objects to JSON. See the JSON and objects documentation for more.

Mounting and linking

The big change in Morepath 0.7 however involves the way mounting works, and how you can link between applications. This introduces a few breaking changes if you were using these features before. The CHANGES document provides documentation that will tell you how to adjust your code.

I'm very happy with the new change. It cleans up several APIs. I believe this makes them both easier to understand while at the same time significantly cleaning up the implementation. It also introduces a powerful new feature for inter-app linking: deferred links.

In brief, Morepath lets you mount one application into another:

import morepath

class RootApp(morepath.App)

class SubApp(morepath.App):

@RootApp.mount(path='sub', app=SubApp)
def mount_subapp():
    return SubApp()

Now the SubApp application, which can be its own whole different thing (its instance is a WSGI application), is mounted under the RootApp application. When you go to /sub, SubApp takes over.

This doesn't work just for simple sub-paths like sub, but also for parameterized paths. Consider this:

class WikiApp(morepath.App):
    def __init__(self, wiki_id):
        self.wiki_id = wiki_id

@UserApp.mount(path='/users/{username}/wiki', app=WikiApp)
def mount_wiki(username):
    return WikiApp(wiki_id=wiki_id_for_username(username)

Here's we've mounted a wiki app into a user app. When you go to /users/foo/wiki, the wiki app for user foo takes over, with its own routes, views, and the like. The wiki app doesn't need to know about the user app, and the user app just needs to know how to mount the wiki app.

Morepath is very good at linking: it knows how to construct a link to an object instance. So, if you want to link to a particular WikiPage instance from within the wiki app, you'd simply write this:

What if you wanted to create a link to a wiki page from the user app? Just linking to the wiki page will fail, as the user app doesn't know how to create links to wiki pages. But you can tell it to create a link to an object in the wiki app explicitly:

wiki_app =, username='foo'), app=wiki_app)

If you are going to write a lot of such links, this can get boring. Morepath introduces a new defer_links directive to help automate this:

def defer_links_wiki_page(app, obj):
    return app.child(WikiApp(obj.wiki_id))

You have told Morepath that when it wants to create a link to a wiki page it should consult a mounted wiki app? Which wiki id to use is determined by inspecting the wiki page object -- it's assumed it knows in which wiki it belongs in this example.

Now you can just write this in the user app to link to wiki pages:

Read the nesting applications documentation for more details.


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