# API

This part of the documentation covers all the interfaces of Flask. For parts where Flask depends on external libraries, we document the most important right here and provide links to the canonical documentation.

## Incoming Request Data

To access incoming request data, you can use the global request object. Flask parses incoming request data for you and gives you access to it through that global object. Internally Flask makes sure that you always get the correct data for the active thread if you are in a multithreaded environment.

This is a proxy. See :ref:notes-on-proxies for more information.

The request object is an instance of a :class:~werkzeug.wrappers.Request subclass and provides all of the attributes Werkzeug defines. This just shows a quick overview of the most important ones.

## Sessions

If you have the :attr:Flask.secret_key set you can use sessions in Flask applications. A session basically makes it possible to remember information from one request to another. The way Flask does this is by using a signed cookie. So the user can look at the session contents, but not modify it unless they know the secret key, so make sure to set that to something complex and unguessable.

To access the current session you can use the :class:session object:

The session object works pretty much like an ordinary dict, with the difference that it keeps track on modifications.

This is a proxy. See :ref:notes-on-proxies for more information.

The following attributes are interesting:

## Session Interface

The session interface provides a simple way to replace the session implementation that Flask is using.

Notice

The PERMANENT_SESSION_LIFETIME config key can also be an integer starting with Flask 0.8. Either catch this down yourself or use the :attr:~flask.Flask.permanent_session_lifetime attribute on the app which converts the result to an integer automatically.

## Application Globals

To share data that is valid for one request only from one function to another, a global variable is not good enough because it would break in threaded environments. Flask provides you with a special object that ensures it is only valid for the active request and that will return different values for each request. In a nutshell: it does the right thing, like it does for :class:request and :class:session.

## JSON Support

Flask uses simplejson for the JSON implementation. Since simplejson is provided both by the standard library as well as extension Flask will try simplejson first and then fall back to the stdlib json module. On top of that it will delegate access to the current application's JSOn encoders and decoders for easier customization.

So for starters instead of doing:

try:
import simplejson as json
except ImportError:
import json


You can instead just do this:

from flask import json


For usage examples, read the :mod:json documentation in the standard lirbary. The following extensions are by default applied to the stdlib's JSON module:

1. datetime objects are serialized as RFC 822 strings.
2. Any object with an __html__ method (like :class:~flask.Markup) will ahve that method called and then the return value is serialized as string.

The :func:~htmlsafe_dumps function of this json module is also available as filter called |tojson in Jinja2. Note that inside script tags no escaping must take place, so make sure to disable escaping with |safe if you intend to use it inside script tags:

<script type=text/javascript>
doSomethingWith({{ user.username|tojson|safe }});
</script>


Note that the |tojson filter escapes forward slashes properly.

## Signals

An alias for :class:blinker.base.Namespace if blinker is available, otherwise a dummy class that creates fake signals. This class is available for Flask extensions that want to provide the same fallback system as Flask itself.

## URL Route Registrations

Generally there are three ways to define rules for the routing system:

1. You can use the :meth:flask.Flask.route decorator.
2. You can use the :meth:flask.Flask.add_url_rule function.
3. You can directly access the underlying Werkzeug routing system which is exposed as :attr:flask.Flask.url_map.

Variable parts in the route can be specified with angular brackets (/user/<username>). By default a variable part in the URL accepts any string without a slash however a different converter can be specified as well by using <converter:name>.

Variable parts are passed to the view function as keyword arguments.

The following converters are available:

 string accepts any text without a slash (the default) int accepts integers float like int but for floating point values path like the default but also accepts slashes

Here are some examples:

@app.route('/')
def index():
pass

@app.route('/<username>')
def show_user(username):
pass

@app.route('/post/<int:post_id>')
def show_post(post_id):
pass


An important detail to keep in mind is how Flask deals with trailing slashes. The idea is to keep each URL unique so the following rules apply:

1. If a rule ends with a slash and is requested without a slash by the user, the user is automatically redirected to the same page with a trailing slash attached.
2. If a rule does not end with a trailing slash and the user requests the page with a trailing slash, a 404 not found is raised.

This is consistent with how web servers deal with static files. This also makes it possible to use relative link targets safely.

You can also define multiple rules for the same function. They have to be unique however. Defaults can also be specified. Here for example is a definition for a URL that accepts an optional page:

@app.route('/users/', defaults={'page': 1})
@app.route('/users/page/<int:page>')
def show_users(page):
pass


This specifies that /users/ will be the URL for page one and /users/page/N will be the URL for page N.

Here are the parameters that :meth:~flask.Flask.route and :meth:~flask.Flask.add_url_rule accept. The only difference is that with the route parameter the view function is defined with the decorator instead of the view_func parameter.

 rule the URL rule as string endpoint the endpoint for the registered URL rule. Flask itself assumes that the name of the view function is the name of the endpoint if not explicitly stated. view_func the function to call when serving a request to the provided endpoint. If this is not provided one can specify the function later by storing it in the :attr:~flask.Flask.view_functions dictionary with the endpoint as key. defaults A dictionary with defaults for this rule. See the example above for how defaults work. subdomain specifies the rule for the subdomain in case subdomain matching is in use. If not specified the default subdomain is assumed. **options the options to be forwarded to the underlying :class:~werkzeug.routing.Rule object. A change to Werkzeug is handling of method options. methods is a list of methods this rule should be limited to (GET, POST etc.). By default a rule just listens for GET (and implicitly HEAD). Starting with Flask 0.6, OPTIONS is implicitly added and handled by the standard request handling. They have to be specified as keyword arguments.

## View Function Options

For internal usage the view functions can have some attributes attached to customize behavior the view function would normally not have control over. The following attributes can be provided optionally to either override some defaults to :meth:~flask.Flask.add_url_rule or general behavior:

• __name__: The name of a function is by default used as endpoint. If endpoint is provided explicitly this value is used. Additionally this will be prefixed with the name of the blueprint by default which cannot be customized from the function itself.
• methods: If methods are not provided when the URL rule is added, Flask will look on the view function object itself is an methods attribute exists. If it does, it will pull the information for the methods from there.
• provide_automatic_options: if this attribute is set Flask will either force enable or disable the automatic implementation of the HTTP OPTIONS response. This can be useful when working with decorators that want to customize the OPTIONS response on a per-view basis.
• required_methods: if this attribute is set, Flask will always add these methods when registering a URL rule even if the methods were explicitly overriden in the route() call.

Full example:

def index():
if request.method == 'OPTIONS':
# custom options handling here
...
return 'Hello World!'
index.provide_automatic_options = False
index.methods = ['GET', 'OPTIONS']

app.add_url_rule('/', index)