Flask-Classy

flask.ext.classy

Flask-Classy is an extension that adds class-based views to Flask. But why?

I ❤ Flask. Like a lot. But sometimes projects get a little big and I need some way of managing and organizing all the different pieces. I know what you're saying: "But what about Blueprints?"

You're right. Blueprints are pretty awesome. But I found that they aren't always enough to encapsulate a specific context the way I need. What I wanted, no what I needed was to be able to group my views into relevant classes each with their own context and behavior. It's also made testing really nifty too.

"OK, I see your point. But can't I just use the base classes in flask.views to do that?"

Well, yes and no. While flask.views.MethodView does provide some of the functionality of flask.ext.classy.FlaskView it doesn't quite complete the picture by supporting methods that aren't part of the typical CRUD operations for a given resource, or make it easy for me to override the route rules for particular view. And while flask.views.View does add some context, it requires a class for each view instead of letting me group very similar views for the same resource into a single class.

"But my projects aren't that big. Can Flask-Classy do anything else for me besides making a big project easier to manage?"

Why yes. It does help a bit with one other thing.

Flask-Classy will automatically generate routes based on the methods in your views, and makes it super simple to override those routes using Flask's familiar decorator syntax.

Installation

Install the extension with:

$ pip install flask-classy

or if you're kickin' it old-school:

$ easy_install flask-classy

Let's see how it works

If you're like me, you probably get a better idea of how to use something when you see it being used. Let's go ahead and create a little app to see how Flask-Classy works:

from flask import Flask
from flask.ext.classy import FlaskView

# we'll make a list to hold some quotes for our app
quotes = [
    "A noble spirit embiggens the smallest man! ~ Jebediah Springfield",
    "If there is a way to do it better... find it. ~ Thomas Edison",
    "No one knows what he can do till he tries. ~ Publilius Syrus"
]

app = Flask(__name__)

class QuotesView(FlaskView):
    def index(self):
        return "<br>".join(quotes)

QuotesView.register(app)

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

Run this app and open your web browser to: http://localhost:5000/quotes/

As you can see, it returns the list of quotes. But what if we just wanted one quote? What would we do then?

class QuotesView(FlaskView):
    def index(self):
        ...

    def get(self, id):
        id = int(id)
        if id < len(quotes) - 1:
            return quotes[id]
        return "Not Found", 404

Now direct your browser to: http://localhost:5000/quotes/1/ and you should see the very poignant quote from the esteemed Mr. Edison.

That's cool and all, but what if we just wanted a random quote? What then? Let's add a random view to our FlaskView:

from random import choice

class QuotesView(FlaskView):
    def index(self):
        ...

    def get(self, id):
        ...

    def random(self):
        return choice(quotes)

And point your browser to: http://localhost:5000/quotes/random/ and see that a random quote is returned each time. Voila!

So by now you must be keenly aware of the fact that you have not defined a single route, but yet routing is obviously taking place. "Is this voodoo?" you ask?

Not at all. Flask-Classy will automatically create routes for any method in a FlaskView that doesn't begin with an underscore character. You can still define your own routes of course, and we'll look at that next.

Using custom routes

So let's pretend that /quotes/random/ is just too unsightly and we must fix it to be something more spectacular forthwith. In a moment of blind inspiration we decide that getting a random quote is on par with receiving a rasher of your favorite porcine delicacy. The new url should be /quotes/word_bacon/ so that everyone knows what a treat they are in for.

from flask.ext.classy import FlaskView, route

class QuotesView(FlaskView):
    def index(self):
        ...

    def get(self, id):
        ...

    @route('/word_bacon/') #<--- Adding route
    def random(self):
        return choice(quotes)

Load up http://localhost/quotes/word_bacon/ in your browser and behold your latest achievement.

The route decorator takes exactly the same parameters as Flask's app.route decorator, so you should feel right at home adding custom routes to any views you create.

So far, all of our urls have been prefixed by that /quotes bit and you have probably deduced that it was derived from the name of your FlaskView instance (minus the "View" suffix, of course.) "That's all well and good," you're saying, "but how do I change that? What if I want my views at the root?" Well, person, I have an answer for you.

Customizing the Route Base

There are 2 ways to customize the base route of a FlaskView. (Well technically there are 3 if you count changing the name of the class but that's hardly a reasonable way to go about it.)

Method 1:

The first method simply requires you to set a route_base attribute on your FlaskView. Suppose we wanted to make our QuotesView handle the root of the web application:

class QuotesView(FlaskView):
    route_base = '/'

    def index(self):
        ...

    def get(self, id):
        ...

    @route('/word_bacon/')
    def random(self):
        ...

Method 2:

The second method is perfect for when you're using app factories, and you need to be able to specify different base routes for different apps. You can specify the route when you register the class with the Flask app instance:

QuotesView.register(app, route_base='/')

The second method will always override the first, so you can use method one, and override it with method two if needed. Sweet!

Special method names

So I guess I have to break the narrative a bit here so I can take some time to talk about Flask-Classy's special method names.

Here's the thing. FlaskView is smart. No, not solving differential equations smart, but let's just say it knows how to put the round peg in the round hole. When you register a FlaskView with an app, FlaskView will look for special methods in your class. Why? Because I care. I know that sometimes you just want things to just work and not have to think about it. Let's look at FlaskView's very special method names:

index

Woah... you've seen this one before! Remember way back at the beginning? Oh nevermind. So index is generally used for home pages and lists of resources. The automatically generated route is:

rule:   '/'
name:   <class name>:index
method: GET

get

Another old familiar friend, get is usually used to retrieve a specific resource. The automatically generated route is:

rule:   '/<id>/'
name:   <class name>:get
method: GET

post

This method is generally used for creating new instances of a resource but can really be used to handle any posted data you want. The automatically generated route is:

rule:   '/'
name:   <class name>:post
method: POST

put

For those of us using REST this one is really helpful. It's generally used to update a specific resource. The automatically generated route is:

rule:   '/<id>/'
name:   <class name>:put
method: PUT

patch

Similar to put, patch is used for updating a resource. Unlike put however you only send the parts of the resource you want changed, instead of doing a complete replacement of the resource. The automatically generated route is:

rule:   '/<id>/'
name:   <class name>:patch
method: PATCH

delete

More RESTfulness. It's the most self explanitory of all the RESTful methods, and it's commonly used to destroy a specific resource. The automatically generated route is:

rule:   '/<id>/'
name:   <class name>:delete
method: DELETE

Your own methods (they're special too!)

And lastly, but not leastly, let's talk about how you can add your own methods (like we did with random back in the day, remember? Good times.) If you add your own methods FlaskView will detect them during registration and register routes for them, whether you've gone and defined your own, or you just want to let FlaskView do it's thing. By default, FlaskView will create a route that is the same as the method name. So if you define a view method in your FlaskView like this:

class SomeView(FlaskView):
    route_base = "root"

    def my_view(self):
        return "Check out my view!"

FlaskView will generate a route like this:

rule:   '/some/my_view/'
name:   SomeView:my_view0
method: GET

"That's fine." you say. "But what if I have a view method with some parameters?" Well FlaskView will try to take care of that for you too. If you were to define another view like this:

class AnotherView(FlaskView):
    route_base = "home"

    def this_view(self, arg1, arg2):
        return "Args: %s, %s" % (arg1, arg2,)

FlaskView would generate a route like this:

rule:   '/home/this_view/<arg1>/<arg2>/'
name:   AnotherView:this_view0
method: GET

One important thing to note, is that FlaskView does not type your parameters, so if you want or need them you'll need to define the route yourself using the @route decorator.

Questions?

Feel free to ping me on twitter @apiguy, or head on over to the github repo at http://github.com/apiguy/flask-classy so you can join the fun.