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context was set up. Because the request context is already bound, the subscriber
can access the request with the standard global proxies such as request.
Example subscriber:
def log_request(sender, **extra):
sender.logger.debug(Request context t is s set t up)
from flask import request_started
request_started.connect(log_request, app)
flask.request_finished
This signal is sent right before the response is sent to the client. It is passed the
response to be sent named response.
Example subscriber:
def log_response(sender, response, **extra):
sender.logger.debug(Request context t is s about to o close down. . 
Response: %s, response)
from flask import request_finished
request_finished.connect(log_response, app)
flask.got_request_exception
This signal is sent when an exception happens during request processing. It is
sent before the standard exception handling kicks in and even in debug mode,
where no exception handling happens. The exception itself is passed to the sub-
scriber as exception.
Example subscriber:
def log_exception(sender, exception, **extra):
sender.logger.debug(Got exception during processing: %s, exception)
from flask import got_request_exception
got_request_exception.connect(log_exception, app)
flask.request_tearing_down
This signal is sent when the request is tearing down. This is always called, even
if an exception is caused. Currently functions listening to this signal are called
after the regular teardown handlers, but this is not something you can rely on.
Example subscriber:
def close_db_connection(sender, **extra):
session.close()
from flask import request_tearing_down
request_tearing_down.connect(close_db_connection, app)
As of Flask 0.9, this will also be passed an exc keyword argument that has a
reference to the exception that caused the teardown if there was one.
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flask.appcontext_tearing_down
This signal is sent when the app context is tearing down. This is always called,
even if an exception is caused. Currently functions listening to this signal are
called after the regular teardown handlers, but this is not something you can
rely on.
Example subscriber:
def close_db_connection(sender, **extra):
session.close()
from flask import appcontext_tearing_down
appcontext_tearing_down.connect(close_db_connection, app)
This will also be passed an exc keyword argument that has a reference to the
exception that caused the teardown if there was one.
flask.appcontext_pushed
This signal is sent when an application context is pushed. The sender is the
application. This is usually useful for unittests in order to temporarily hook in
information. For instance it can be used to set a resource early onto the g object.
Example usage:
from contextlib import contextmanager
from flask import appcontext_pushed
@contextmanager
def user_set(app, user):
def handler(sender, **kwargs):
g.user = user
with appcontext_pushed.connected_to(handler, app):
yield
And in the testcode:
def test_user_me(self):
with user_set(app, john):
= app.test_client()
resp = c.get(/users/me)
assert resp.data == username=john
New in version 0.10.
appcontext_popped
This signal is sent when an application context is popped. The sender is the
application. This usually falls in line with the appcontext_tearing_down signal.
New in version 0.10.
flask.message_flashed
This signal is sent when the application is flashing a message. The messages is
sent as message keyword argument and the category as category.
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Example subscriber:
recorded = []
def record(sender, message, , category, **extra):
recorded.append((message, category))
from flask import message_flashed
message_flashed.connect(record, app)
New in version 0.10.
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CHAPTER
TWELVE
PLUGGABLE VIEWS
New in version 0.7.
Flask 0.7 introduces pluggable views inspired by the genericviews fromDjango which
are based on classes instead of functions. The main intention is that you can replace
parts of the implementations and this way have customizable pluggable views.
12.1 Basic Principle
Consider you have a functionthat loads a list of objects fromthe database and renders
into a template:
@app.route(/users/)
def show_users(page):
users = User.query.all()
return render_template(users.html, users=users)
This is simple and flexible, but if you want to provide this view in a generic fashion
that can be adapted to other models and templates as well you might want more flex-
ibility. This is where pluggable class-based views come into place. As the first step to
convert this into a class based view you would do this:
from flask.views import View
class ShowUsers(View):
def dispatch_request(self):
users = User.query.all()
return render_template(users.html, objects=users)
app.add_url_rule(/users/, view_func=ShowUsers.as_view(show_users))
As you canseewhatyou have to do is to create a subclass of flask.views.View and im-
plement dispatch_request(). Then we have to convert that class into an actual view
function by using the as_view() class method. The string you pass to that function is
the name of the endpoint that view will then have. But this by itself is not helpful, so
let’s refactor the code a bit:
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from flask.views import View
class ListView(View):
def get_template_name(self):
raise NotImplementedError()
def render_template(self, context):
return render_template(self.get_template_name(), **context)
def dispatch_request(self):
context = {objectsself.get_objects()}
return self.render_template(context)
class UserView(ListView):
def get_template_name(self):
return users.html
def get_objects(self):
return User.query.all()
This of course is not that helpful for such a small example, but it’s good enough to
explain the basic principle. When you have a class-based view the question comes up
what self points to. The way this works is that whenever the request is dispatched a
new instance of the class is created and the dispatch_request() method is called with
the parameters from the URL rule. The class itself is instantiated with the parameters
passed to the as_view() function. For instance you can write a class like this:
class RenderTemplateView(View):
def __init__(self, template_name):
self.template_name template_name
def dispatch_request(self):
return render_template(self.template_name)
And then you can register it like this:
app.add_url_rule(/about, view_func=RenderTemplateView.as_view(
about_page, template_name=about.html))
12.2 Method Hints
Pluggable views are attached to the application like a regular function by either using
route() or better add_url_rule(). That however also means that you would have to
provide the names of the HTTP methods the view supports when you attach this. In
order to move that information to the class you can provide a methods attribute that
has this information:
76
class MyView(View):
methods = [GETPOST]
def dispatch_request(self):
if request.method == POST:
...
...
app.add_url_rule(/myview, view_func=MyView.as_view(myview))
12.3 Method Based Dispatching
For RESTful APIs it’s especially helpful to execute a different function for each HTTP
method. With the flask.views.MethodView you caneasily do that. EachHTTP method
maps to a function with the same name (just in lowercase):
from flask.views import MethodView
class UserAPI(MethodView):
def get(self):
users = User.query.all()
...
def post(self):
user = User.from_form_data(request.form)
...
app.add_url_rule(/users/, view_func=UserAPI.as_view(users))
That way you also don’t have to provide the methods attribute. It’s automatically set
based on the methods defined in the class.
12.4 Decorating Views
Since the view class itself is not the view function that is added to the routing system
it does not make much sense to decorate the class itself. Instead you either have to
decorate the return value of as_view() by hand:
def user_required(f):
"""Checks whether user is logged in or raises error 401."""
def decorator(*args, **kwargs):
if not g.user:
abort(401)
return f(*args, **kwargs)
return decorator
77
view = user_required(UserAPI.as_view(users))
app.add_url_rule(/users/, view_func=view)
Starting with Flask 0.8 there is also an alternative way where you can specify a list of
decorators to apply in the class declaration:
class UserAPI(MethodView):
decorators = [user_required]
Due to the implicit self from the caller’s perspective you cannot use regular view dec-
orators on the individual methods of the view however, keep this in mind.
12.5 Method Views for APIs
Web APIs are often working very closely with HTTP verbs so it makes a lot of sense
to implement such an API based on the MethodView. That said, you will notice that the
API will require differentURL rules that go to the same method view most of the time.
For instance consider that you are exposing a user object on the web:
URL
Method
Description
/users/
GET
Gives a list of all users
/users/
POST
Creates a new user
/users/<id>
GET
Shows a single user
/users/<id>
PUT
Updates a single user
/users/<id>
DELETE
Deletes a single user
So how would you go about doing that with the MethodView? The trick is to take
advantage of the fact that you can provide multiple rules to the same view.
Let’s assume for the moment the view would look like this:
class UserAPI(MethodView):
def get(self, user_id):
if user_id is None:
# return a list of users
pass
else:
# expose a single user
pass
def post(self):
# create a new user
pass
def delete(self, user_id):
# delete a single user
pass
78
def put(self, user_id):
# update a single user
pass
So how do we hook this up with the routing system? By adding two rules and explic-
itly mentioning the methods for each:
user_view = UserAPI.as_view(user_api)
app.add_url_rule(/users/, defaults={user_idNone},
view_func=user_view, methods=[GET,])
app.add_url_rule(/users/, view_func=user_view, methods=[POST,])
app.add_url_rule(/users/<int:user_id>, view_func=user_view,
methods=[GETPUTDELETE])
If you have a lot of APIs that look similar you can refactor that registration code:
def register_api(view, endpoint, url, , pk=id, pk_type=int):
view_func = view.as_view(endpoint)
app.add_url_rule(url, defaults={pk: None},
view_func=view_func, methods=[GET,])
app.add_url_rule(url, view_func=view_func, methods=[POST,])
app.add_url_rule(%s<%s:%s> % (url, pk_type, pk), view_func=view_func,
methods=[GETPUTDELETE])
register_api(UserAPI, user_api/users/, pk=user_id)
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