Flask-Classy

.. module:: 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 some other things.

For example, 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]
        else:
            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. Voilà!

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:5000/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.

Note

If you want to use other decorators with your views, you'll need to make sure that the @route decorators are first.

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.

Flask-Classy's way of talking about "routes"

OK, So I don't want to start inventing a new language (actually I'd love to invent a new language, just not right now...) for talking about URLs, but since Flask-Classy gives you a lot of flexibility in customizing your routes we might as well make sure we're talking about the same things when we talk about what you can do.

What you see below is a route comprised of a route prefix, and a route base:

/neat_prefix/great_base/

The prefix /neat_prefix/ is only included if you explicitly specify a a prefix for the FlaskView, otherwise no prefix will be applied.

The route base /great_base/ will always exist, either because it was inferred automatically from the name of the FlaskView class, or because you specified a route base to use.

Specifying a Route Prefix

A route prefix is a great way to define a common base to urls. For example lets say you had a bunch of views that were all part of your application's api system.

You could write custom route bases for all of them, but if you want to use Flask Classy's (admittedly amazing) automatic route generation stuff you'll lose the part where it infers the route base from the name of the class.

A better choice is to use a route prefix.

You can specify a route prefix either as an attribute of the FlaskView, or when you register the FlaskView with the application.

As an attribute:

Using an attribute is a great way to define a default prefix, as you can always override this value when you register the FlaskView with your app:

class BurgundyView(FlaskView):

route_prefix = '/colors/'

def index(self):

...

When registering:

Alternatively (or additionally, if you like) you can specify a route prefix when you register the route with your app:

BurgundyView.register(app, route_prefix='/redish_colors/')

And this will override any route prefixes set on the FlaskView class itself.

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!

Using multiple routes for a single view

What happens when you need to apply more than one route to a specific view (for what it's worth, Flask core developer Armin Ronacher says doing that is a bad idea). But since you're so determined let's see how to do that anyway.

So let's say you add the following routes to one of your views:

class QuotesView(FlaskView):
    route_base = '/'

    @route('/quote/<id>')
    @route('/quote/show/<id>')
    def show_quote(self, id):
        ...

That would end up generating the following 2 routes:

rule	/quote/<id>
endpoint	QuotesView:show_quote_1
method	GET

rule	/quote/show/<id>
endpoint	QuotesView:show_quote_0
method	GET

"Oh weird! What's with all the _0 and _1 stuff?" you ask in disgust. Well first I want to know how you managed to pronounce _0. But really the reason is that since there is more than one route, an index is added to prevent an endpoint collision. This differs from the default behavior of Flask, which allows you to create collisions.

Specify your own damn endpoint

So you don't like the nifty indexing trick? Well fine then. I guess you can go ahead and specify your own endpoint if you like but that's only because I like you.

class QuotesView(FlaskView):
    route_base = '/'

    @route('/quote/<id>', endpoint='show_quote')
    @route('/quote/show/<id>')
    def show_quote(self, id):
        ...

Will generate the following routes:

rule	/quote/<id>
endpoint	show_quote
method	GET

rule	/quote/show/<id>
endpoint	QuotesView:show_quote_0
method	GET

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	/
endpoint	<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>
endpoint	<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	/
endpoint	<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>
endpoint	<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>
endpoint	<class name>:patch
method	PATCH

delete

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

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

url_for art thou, Romeo?

Sorry that's a terrible name for a section header, but naming things is what am the least skilled at, so please bear with me.

Once you've got your FlaskView registered, you'll probably want to be able to get the urls for it in your templates and redirects and whatnot. Flask ships with the awesome url_for function that does an excellent job of turning a function name into a url that maps to it. You can use url_for with Flask-Classy by using the format "<Class name>:<method name>". Let's look at an example:

class DuckyView(FlaskView):
    def index(self):
        return "Duckies!"

    def get(self, name):
        return "Duck %s" % name

    @route('/do_duck_stuff', endpoint='do_duck_stuff')
    def post(self):
        return "Um... Quack?"

In this example, you can get a url to the index method using:

url_for("DuckyView:index")

And you can get a url to the get method using:

url_for("DuckyView:get", name="Howard")

And for that view with the custom endpoint defined?:

url_for("do_duck_stuff")

Note

Notice that the custom endpoint does not get prefixed with the class name like the auto-generated endpoints. When you define a custom endpoint, we hand that over to Flask in it's original, unaltered form.

Your own methods (they're special too!)

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/
endpoint	SomeView:my_view
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>
endpoint	AnotherView:this_view
method	GET

Note

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.

Decorating Tips

So if you're like me (and who isn't?), you think that FlaskViews are frickin' beautiful. But once you've moved in it's nice to add a little personal touch, don't you think?

Of course I'm talking about decorators. The Flask ecosystem is full of excellent extensions that allow you to customize a view's behavior simply by adding a decorator, and you can use them with your FlaskView's too.:

class BetterButterView(FlaskView):

    @login_required  # Ain't it pretty?
    def super_secret(self):
        return "It's a secret to everyone."

But what about when you want to add a decorator to every method in your FlaskView? All you need to do is add a decorators attribute to the class definition with a list of decorators you want applied to every method and Flask-Classy will take care of the rest:

class WhataGreatView(FlaskView):
    decorators = [login_required]

    def this_is_secret(self):
        return "If you see this, you're logged in."

    def so_is_this(self):
        return "Looking at me? I guess you're logged in."

Before and After

Hey, remember that time when you made that big 'ol Flask app and then had those @app.before_reqeust and @app.after_request decorated methods? Remember how you only wanted some of them to run for certain views so you had all those if view == the_one_I_care_about: statements and stuff?

Yuck.

I've been there too, and I think you might like how Flask-Classy addresses this very touchy issue. FlaskView will look for wrapper methods when your request is being processed so that you can create more fine grained "before and after" processing methods.

Wrap all the views in a FlaskView

So there you are, eating a delicious Strawberry Frosted Pop Tart one morning, thinking about the awesome Flask-Classy app you deployed the night before during one of your late night hackathons and it hits you:

"Tracking! I need to track those widgets!"

No doubt it's an inspired thought, but in this case it was a tragic oversight. You realize now how lucky it was that you chose to use Flask-Classy because adding tracking is going to be a snap:

from flask.ext.classy import FlaskView
from made_up_tracking import track_it

class WidgetsView(FlaskView):

    def before_request(self, name):
        track_it("something is happening to a widget")

    def after_request(self, name, response):
        track_it("something happened to a widget")
        return response

    def post(self):
        ...

    def get(self, id):
        ...

Whew. Crisis averted, am I right? So you go about your day and at lunch time you hit your favorite Bacon Sandwich place and start daydreaming about your life as a rockstar Flask-Classy consultant when suddenly:

"I really only care about when widgets are created and retrieved!"

Wrap only specific views

Yep, you've got a granularity problem. Not to worry though because Flask-Classy is happy to let you know that it has smart wrapper methods too. Let's say for example you wanted to run something before the index view runs? Just create a method called before_index and Flask-Classy will make sure it gets run only before that view. (as you have guessed by now, after_index will be run only after the index view).

from flask.ext.classy import FlaskView
from made_up_tracking import track_it

class WidgetsView(FlaskView):

    # Will be run before the 'get' view
    def before_get(self):
        track_it("a widget is being accessed")

    # Will be run before the 'post' view
    def after_post(self, response):
        track_it("a widget was created")
        return response

    def post(self):
        ...

    def get(self, id):
        ...

The View Wrappin' Method List

Just to be certain, let's go ahead and review the methods you can write to wrap your views:

before_request(self, name, *args, *kwargs)

Will be called before any view in this FlaskView is called.

name:	The name of the view that's about to be called.
*args:	Any arguments that will be passed to the view.
**kwargs:	Any keyword arguments that will be passed to the view.

before_<view_method>(self, *args, **kwargs)

Will be called before the view specified <view_method> is called.

*args:	Any arguments that will be passed to the view.
**kwargs:	Any keyword arguments that will be passed to the view.

after_request(self, name, response)

Will be called after any view in this FlaskView is called. You must return either the passed in response or a new response.

name:	The name of the view that was called.
resposne:	The response produced after calling the view.

after_<view_method>(self, response)

Will be called after the <view_method> is called. You must return either the passed in response or a new response.

resposne:	The response produced after calling the view.

Order of Wrapped Method Execution

Wrapper methods are called in the same order every time. "How predictable." you're thinking. (You're starting to sound like my ex, sheesh.) I prefer the term reliable.

1. Any method registered with @app.before_request
2. FlaskView's before_request method
3. FlaskView's before_<view_method> method
4. The actual view method
5. FlaskView's after_<view_method> method
6. FlaskView's after_request method
7. Any method registered with @app.after_request

Subdomains (getting advanced 'n stuff)

By now, you've built a few hundred Flask apps using Flask-Classy and you probably think you're an expert. But not until you've tried the snazzy Subdomains feature my friend.

Flask-Classy allows you to specify a subdomain to be used when registering routes for your FlaskViews. While the usefulness of this feature is probably apparent to many of you, let's go ahead and take a look at one of the many facilitative use cases.

Suppose you've got a sweet api you're porting over from a legacy app and in the migration you want to clean things up a bit and start using a subdomain like api.socool.biz instead of the old way of accessing it using api at the root of the path like socool.biz/api. The only catch, of course, is that you have api clients still using that old path based method. What is a hard working developer like you to do?

Thanks to Flask and Flask-Classy you have some options. There are two easy ways you can choose from to tell Flask-Classy which subdomains your FlaskView should respond to.

Let's see both methods so you can choose which one works best for your application.

The Define-During-Registration Method

Probably the most flexible method, you can define which subdomains you want to support at the same time you're registering your views:

# views.py

from flask.ext.classy import FlaskView

class CoolApiView(FlaskView):

    def index(self):
        return "API stuff"

# main.py

from flask import Flask
from views import CoolApiView

app = Flask(__name__)
app.config['SERVER_NAME'] = 'socool.biz'

# This one matches urls like: http://socool.biz/api/...
CoolApiView.register(app, route_base='/api/')

# This one matches urls like: http://api.socool.biz/...
CoolApiView.register(app, route_base='/', subdomain='api')

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

The Explicit-Define-In-The-FlaskView Method

Using this method, you can explicitly define a subdomain as an attribute of the FlaskView subclass:

# views.py

from flask.ext.classy import FlaskView

class CoolApiView(FlaskView):
    subdomain = "api"

    def index(self):
        return "API Stuff"

# main.py

from flask import Flask
from views import CoolApiView

app = Flask(__name__)
app.config['SERVER_NAME'] = 'socool.biz'

# This one matches urls like: http://socool.biz/api/...
CoolApiView.register(app, route_base='/api/', subdomain='')

# This one matches urls like: http://api.socool.biz/...
CoolApiView.register(app, route_base="/")

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

As you can see here, specifying the subdomain to the register method will override the explicit subdomain attribute set inside the class.

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.

API

.. autoclass:: flask.ext.classy.FlaskView
    :members:

.. autofunction:: flask.ext.classy.route

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© Copyright 2013 by Freedom Dumlao, Follow Me @apiguy