Posterous has been getting a lot of attention lately. Its simplicity appeals to a wide range of users and more and more users are switching over from other well-established blogging platforms such as Blogger or WordPress. In this article, we will have a look at how you can replicate the Posterous functionality on App Engine.

In case you don’t know what Posterous is, it’s very simple. It’s a blogging tool like Google’s Bloggeror Wordpress.com. What makes it different from the other services is its ridiculous simplicity. With Posterous, you don’t need to create an account. That’s because you create new posts by emailing your post to Posterous. It creates links for you, images sent as attachments will be transformed into galleries, etc.

I like Posterous, but my website has a lot of custom programming on the back-end so I’m very hesitant to switch over. And besides, I like to know how things work behind the scenes. I thought it might be fun to create a system similar to Posterous’ for my own blog which is, of course, hosted on App Engine.

Here is what we are going to do. I want to be able to send an email to my own blog and have it turn it into a post and publish it to the blog.

OK, let’s start with the post model:

class Post(db.Model):
    title = db.StringProperty()
    body = db.TextProperty()
    added = db.DateTimeProperty(auto_now_add=True)
    author = db.StringProperty()

Very straight forward. You have your title, body, author and when the post was published.

In order to enable incoming email, you need to add a couple of lines of code to your app.yaml file. In addition to your regular handlers, add the following:

inbound_services:
- mail

handlers:
- url: /_ah/mail/.+
  script: main.py

The first line enables incoming email for your application. The second part is the important part. On App Engine, an incoming email message is processed as a HTTP POST request. Since it’s a regular HTTP request, we will need a handler for it in the app.yaml file. You have several options here. You can create a catch-all handler for all incoming email addresses (like I’ve done above), or create seperate handlers for different addresses.

The email address that we will use is in the following format: your_string@appid.appspotmail.com. You should substitute the appid with your app’s ID. The string before the ‘@’ symbol can be set to anything you want.

With this out of the way, we are ready to write the actual email handler. This will go into your main.py file which you defined in the app.yaml.

First, some imports:

import email
from google.appengine.ext.webapp.mail_handlers import InboundMailHandler

Then, you add the following to the list of URL mappings in the instantiation of the application class.

application = webapp.WSGIApplication([
    ('/', Index),
    EmailHandler.mapping()
    ], debug=True)

Here EmailHandler is the request handler class that will handle the incoming email. The mapping() method will map all of the addresses and send all of them to this handler class. It’s just a convenience method, no magic here.

Now, finally, onto the actual handler:

class EmailHandler(InboundMailHandler):
    def receive(self, mail_message):
        post = Post()
        post.title = mail_message.subject
        for content_type, body in mail_message.bodies():
            post.body = body.decode()
        post.author = 'John Smith'
        post.put()

This is actually very simple. The incoming email message is saved in the mail_message variable and you can access all of the usual email metadata as its properties (e.g. mail_message.sender). So, we create a new post, take the email’s subject and set it as the post’s title. The bodies() method extracts the body of the email and the decode() function will decode the actual body. Then we set the author and save the post in the datastore.

Often you will want to include a link in your post, or create a list. This is easily accomplished with HTML tags. However, HTML tags are a pain, so you might want to use something like Markdown.

Adding markdown support is super easy. Download the Python Markdown library and put the markdown2.py file in your app’s root directory. Then import it in your main.py file.

import markdown2

And change the following line:

post.body = body.decode()

to this:

post.body = markdown2.markdown(body.decode())

And that’s it!

Conclusion

This is a very simple yet effective technique and it will allow you to create post from anywhere. I hope you’ve enjoyed the post. Let me know if you have any suggestions on how to improve it.

Code

The complete code for this app is available on Github.