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# 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.

## Application Object

## Blueprint Objects

## Incoming Request Data

class `flask.``request`

  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 Notes On Proxies for more information.

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

## Response Objects

## Sessions

If you have the `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 `session` object:

class `flask.``session`

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

  This is a proxy. See Notes On Proxies for more information.

  The following attributes are interesting:

  `new`

    True if the session is new, False otherwise.

  `modified`

    True if the session object detected a modification. Be advised that modifications on mutable structures are not picked up automatically, in that situation you have to explicitly set the attribute to True yourself. Here an example:

        # this change is not picked up because a mutable object (here
        # a list) is changed.
        session['objects'].append(42)
        # so mark it as modified yourself
        session.modified = True

  `permanent`

    If set to True the session lives for `permanent_session_lifetime` seconds. The default is 31 days. If set to False (which is the default) the session will be deleted when the user closes the browser.

## Session Interface

New in version 0.8.

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 `permanent_session_lifetime` attribute on the app which converts the result to an integer automatically.

## Test Client

## 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 `request` and `session`.

`flask.``g`

  Just store on this whatever you want. For example a database connection or the user that is currently logged in.

  Starting with Flask 0.10 this is stored on the application context and no longer on the request context which means it becomes available if only the application context is bound and not yet a request. This is especially useful when combined with the Faking Resources and Context pattern for testing.

  Additionally as of 0.10 you can use the `get()` method to get an attribute or None (or the second argument) if it’s not set. These two usages are now equivalent:

      user = getattr(flask.g, 'user', None)
      user = flask.g.get('user', None)

  It’s now also possible to use the `in` operator on it to see if an attribute is defined and it yields all keys on iteration.

  This is a proxy. See Notes On Proxies for more information.

## Useful Functions and Classes

`flask.``current_app`

  Points to the application handling the request. This is useful for extensions that want to support multiple applications running side by side. This is powered by the application context and not by the request context, so you can change the value of this proxy by using the `app_context()` method.

  This is a proxy. See Notes On Proxies for more information.

`flask.``abort`(code)

  Raises an `HTTPException` for the given status code. For example to abort request handling with a page not found exception, you would call `abort(404)`.

  Parameters: | code – the HTTP error code.

## Message Flashing

## 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 `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 `Markup`) will ahve that method called and then the return value is serialized as string.

The `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 unless you are using Flask 0.10 which implies that:

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

## Template Rendering

## Configuration

## Extensions

`flask.``ext`

  This module acts as redirect import module to Flask extensions. It was added in 0.8 as the canonical way to import Flask extensions and makes it possible for us to have more flexibility in how we distribute extensions.

  If you want to use an extension named “Flask-Foo” you would import it from `ext` as follows:

      from flask.ext import foo

  New in version 0.8.

## Stream Helpers

## Useful Internals

`flask.``_request_ctx_stack`

  The internal `LocalStack` that is used to implement all the context local objects used in Flask. This is a documented instance and can be used by extensions and application code but the use is discouraged in general.

  The following attributes are always present on each layer of the stack:

  app
    the active Flask application.
  url_adapter
    the URL adapter that was used to match the request.
  request
    the current request object.
  session
    the active session object.
  g
    an object with all the attributes of the `flask.g` object.
  flashes
    an internal cache for the flashed messages.

  Example usage:

      from flask import _request_ctx_stack

      def get_session():
          ctx = _request_ctx_stack.top
          if ctx is not None:
              return ctx.session

`flask.``_app_ctx_stack`

  Works similar to the request context but only binds the application. This is mainly there for extensions to store data.

  New in version 0.9.

## Signals

New in version 0.6.

`flask.``signals_available`

  True if the signalling system is available. This is the case when blinker is installed.

`flask.``template_rendered`

  This signal is sent when a template was successfully rendered. The signal is invoked with the instance of the template as template and the context as dictionary (named context).

`flask.``request_started`

  This signal is sent before any request processing started but when the request 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`.

`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.

`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 subscriber as exception.

`flask.``request_tearing_down`

  This signal is sent when the application is tearing down the request. This is always called, even if an error happened. An exc keyword argument is passed with the exception that caused the teardown.

  Changed in version 0.9: The exc parameter was added.

`flask.``appcontext_tearing_down`

  This signal is sent when the application is tearing down the application context. This is always called, even if an error happened. An exc keyword argument is passed with the exception that caused the teardown. The sender is the application.

`flask.``appcontext_pushed`

  This signal is sent when an application context is pushed. The sender is the application.

  New in version 0.10.

`flask.``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.

  New in version 0.10.

class `flask.signals.``Namespace`

  An alias for `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.

  `signal`(name, doc=None)

    Creates a new signal for this namespace if blinker is available, otherwise returns a fake signal that has a send method that will do nothing but will fail with a `RuntimeError` for all other operations, including connecting.

## Class-Based Views

New in version 0.7.

## URL Route Registrations

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

1. You can use the `flask.Flask.route()` decorator.
2. You can use the `flask.Flask.add_url_rule()` function.
3. You can directly access the underlying Werkzeug routing system which is exposed as `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 `route()` and `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 `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 `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 `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)

New in version 0.8: The provide_automatic_options functionality was added.